It depends on what you mean by "useful". DB was not an ensemble cast story - it was Goku's story. He went around pummeling the armies and villains. The other heroes maybe did something here and there in the regular arc but their best place to show off was in the tournament.
So yeah, the heroes weren't useful in that they were going around defeating the Red Ribbon Army or King Piccolo but they weren't worthless either. They were all impressive fighters in their own right.
In fact, now I think on it, the tournaments really helped a lot of the side-characters look good even if they weren't actually as good as Goku. Goku vs. Krillin? Yamcha vs. Tenshinhan? Neither Krillin or Yamcha stood a cahnce but neither fight was so bad that they came out of it looking like garbage.
That's the difference between DB Part 1 and Part 2 - all the side-characters just looked pathetic.
So you're saying that you just miss the tournaments? Because in all of the other arcs they were instant fodder and did nothing useful. In Z, Tien and Krillin got to actually do important things, and the humans did get a tournament-like moment to shine in the Ginyu Force filler, and against the Saibamen.
I wouldn't say so. Krillin usually looked good- in fact he's one of my favorite characters simply because of his massive titanium balls, when he takes on and lasts against opponents way stronger than him. Piccolo is of course a badass, and he also gets his time to shine (and once again, he also gets a good deal in the movies), while the Saiyans actually help balance out the scale because it shows someone other than Goku do something. For example, even though I don't like the Cell Saga, I do like how everyone got to do something and it's just not just a Goku-fest like the original Dragon Ball. Who defeats Cyborg Freeza? Not Goku, but Trunks. Who defeats 19 and 20? Not Goku, but Vegeta and Piccolo. Who fights Androids 17 and 18 evenly? Not Goku, but Vegeta and Piccolo again (more so the latter). Who defeats the first forms of the main villain, Cell? Not Goku, but Piccolo, then Vegeta and Trunks. Who actually stands and fights against the Cell Juniors? Again, Piccolo, Trunks, and Vegeta, while Goku was on the ground. Who finally defeats Cell? Not Goku, but Gohan, while Goku actually got killed by Cell.
The Buu arc continues this, and it was going good until the end, where it became a Goku-fest again.
Don't get me wrong, I do wish that the non-Saiyans stayed relevant and got to do more things (part of the reason why I labelled Vegeta a kill stealer above), but the original Dragon Ball wasn't really any better about this...
Also, your power tier is really wrong. Krillin is stronger than Tien and the Supreme Kai is WAY above the androids. Though I think you weren't trying to represent a power tier, since you also have Freeza above Androids 16, 17 and 18, and Future Trunks above some form of Buu...