The importance of the base form

[Doctor.]

Toriyama keeps changing his mind on whether training the base form is better than training the Super Saiyan forms. Let's take a look at how the two states are used in comparison to each-other after Super Saiyan's introduction.

Cell arc: The Super Saiyan state is used predominantly throughout the entire arc. The base state is almost never seen. This is likely to save time inking, as that was one of the original motives behind the Super Saiyan design. The SS prevalence over the base form hits an extreme point in the Cell Games, where Super Saiyan is treated like a normal state from there on. Whether this decision was too motivated by laziness or motivated purely by narrative reasons, or maybe a mixture of both, is up to the imagination.

Boo arc: Super Saiyan gradually loses its importance here. The base state is shown a lot more and Gohan's ultimate form seems like a way for Toriyama to eliminate Super Saiyan forms all together, perhaps because by this point they started getting too convoluted for his taste. Even at the 28th Budokai, Goku never once transforms.

Battle of Gods and Resurrection 'F': Toriyama decides to restrict Goku to two forms: a super powerful base form and SSB. Interesting that even though Toriyama takes this conservative approach to SS forms in these movies, Gohan loses his super simple ultimate form and returns to the Super Saiyan forms, perhaps to not confuse his ultimate state with Goku and Vegeta's new base forms. But, again, Toriyama seems to have this idea that the base form should be important in the series. Until...

Champa arc: He goes back to the golden Super Saiyan forms once again. Though the base form still has a lot of battles from here on, the fact that the SS forms are back automatically decreases its power by comparison, taking away from that "super powerful base" idea that Toriyama had planned for the movies. Maybe because he couldn't create fair match-ups for such a powerful Goku or maybe because the Super Saiyan forms are just too marketable.

Tournament of Power: The key visual released shows Goku with a new form, one that is very similar to his base state, though the hairstyle does differ a bit. Gohan also suspiciously gets his ultimate form back at the same time (though I more-so suspect this to be Toei-exclusive, I guess we'll have to wait on the manga for that one).

So, what gives? Why does Toriyama keep implementing this idea that the base form is a big deal after basically telling us that it's irrelevant in the Cell arc, and tries to limit the Saiyans to a very conservative number of transformations, only for him to backtrack on that idea the very next arc? Will Goku be restricted to 1/2 forms after he gets his new power-up? Or will Super Saiyan forms somehow find a way to become relevant again?

[ekrolo2]

I wanna say golden SS was brought back to show case that Goku & Vegeta aren't as powerful as the movies implied them to be but that idea was already dubious at best since there is no 70% line from Beerus or Whis like BoG the movie did.

I believe Toriyama got ideas from Toei for the Black arc, specifically to make Black an evil Goku, so the most likely answer could be him letting all the past SS forms, sans 4, return just for marketing sake and why the F outfits are retired.

[Doctor.]

I wanna say golden SS was brought back to show case that Goku & Vegeta aren't as powerful as the movies implied them to be but that idea was already dubious at best since there is no 70% line from Beerus or Whis like BoG the movie did.

I believe Toriyama got ideas from Toei for the Black arc, specifically to make Black an evil Goku, so the most likely answer could be him letting all the past SS forms, sans 4, return just for marketing sake and why the F outfits are retired.

He wrote the Champa arc on his own though, and the golden SS form was already back there.

[ekrolo2]

I wanna say golden SS was brought back to show case that Goku & Vegeta aren't as powerful as the movies implied them to be but that idea was already dubious at best since there is no 70% line from Beerus or Whis like BoG the movie did.

I believe Toriyama got ideas from Toei for the Black arc, specifically to make Black an evil Goku, so the most likely answer could be him letting all the past SS forms, sans 4, return just for marketing sake and why the F outfits are retired.

He wrote the Champa arc on his own though, and the golden SS form was already back there.

No, I'm saying the golden SS forms coming back was a sort of precedent for the Black arc with regards to Toriyama letting other people influence what he wanted to do.

[LightBing]

There's a constant which is constantly broken and revived. Mr.Toriyama's ideal to simplify the forms into one or two stages. This is there from the start. The Cell Arc is him introducing the idea by exploring the grades transformations and telling us that Super Saiyan in it's most simplistic way is the correct way.
Even Gohan's Super Saiyan 2 wasn't supposed to be a new widespread transformation, just a visualization of his unique potential.

I must underline that base form is irrelevant until the Battle of the Gods. The frequency of it's appearances is related to the opponents faced, nothing more.

Now Mr.Toriyama introduced this idea in the Cell Arc only to break it shortly after, officially introducing SSJ2 and going one step forward with SSJ3. With some unfortunate character regression, in my opinion. Goku's SSJ3 is contradictory to his whole reasoning for mastering SSJ, a logical in-character progression would be to do what Future Trunks did in Super and evolve SSJ2, instead of picking the inferior form.

I digress; Gohan's Ultimate form is once again Mr.Toriyama returning to simplified, easily parsed forms.

In Battle of Gods he expands this to Goku and later Vegeta. His ideal once again restored, one of the phases being base is just due to recognition. It's the starting point, I don't believe Mr.Toriyama has any strong opinions about it being specifically base at all.
Gohan weirdly being SSJ, I attribute to either forgetfulness or his unwillingness to write the character and weakening him like he did in RoF.

What made him change his mind in Super is most likely in-part pressure/suggestion to reintroduce SSJ forms due marketing. I also believe that his simplicity worked in the movies, where the opponents were Juggernauts, in Super not so much. In fact It would work against it's purpose.
There was a need to have the Golden Forms to better perceive the power-scale, now that Super forced him to introduce a wide range of opponents to Goku. Mr.Toriyama likes to have visual queues to explain things.
These two factors are the reasons for the change, what weighted in more I can't say.

TL;DR
Basically it has nothing to do with the Base form. It's just Mr.Toriyama simplifying the forms and going against it and doing again it until we reached the current juncture. The fact that Base form is commonly in the mix it's because it's by far the most recognizable state the characters will ever have and the de-facto starting point.

[precita]

If anything this reminds me what Toei did in the older movies. Anyone remember in the movies Goku would constantly fight in his base form and allow himself to get all beat up before he would transform into Super Saiyan? I remember back in the old days pre-Super people used to constantly call the Toei writers out on this, how movie Goku wouldn't power up right away whereas Toriyama's Goku would.

However now that Toei's writers themselves are writing the individual Super eps, they've gone int his direction.