Sorry for the mini-necro, but I watched Movie 8 for the first time in a while yesterday and actually thought this over when it was finished:
I don't know how familiar you guys are with Western comics, but in the early 1990's DC introduced two major overpowered villains to hurt their flagship icons, Batman and Superman: those characters were
Bane and
Doomsday, respectively. The storylines both characters were used in were very similar, and both were of a kind with Broly's first appearance - Bane/Doomsday appeared out of nowhere, having not ever being so much as hinted to exist, then effortlessly crippled/killed Batman/Superman before being relegated to jobber status for the rest of their existence. Like Broly, these characters had the gimmick of getting more powerful the longer they fought: Bane could constantly jack himself up on steroids, and Doomsday couldn't be killed the same way twice. And like Broly, their existence bitterly divides the fandom, with some deriding them as one-note powerhouses, and others liking them for that very same reason. And again like Broly, these two characters were easily at the top in terms of power during their first introduction, while successive appearances watered down their basic concepts to the point where they were severely depowered and virtually unrecognizable later on.
The reason I bring this up is because there are parallels with the reactions of both the DBZ fandom and the comics fandom to this type of character which TVTropes calls the
Generic Doomsday Villain. This type of character is more a plot device than a fully-recognized character in its own right, being created when the producers need a way to demonstrate a benchmark beyond which the main characters in their current form cannot go, and then re-introduced when the main characters have
exceeded that benchmark and the producers need to show that, too, either by having the main characters now effortlessly defeat him (Broly in Movie 10, Bane when facing off against the "new" Batman) or by jobbing him out to a new villain to hype them up (Doomsday getting reduced to a skeleton by Imperiex).
This type of character tends to drive sales and introduce a lot of new people to a given fanbase, especially because major arcs can be built around them with very little effort. And old-school fans tend to react negatively to the types of new fans these characters bring in. Broly was the
only DBZ movie villain I knew about in middle school because of word-of-mouth, and this was before any of the movies he featured in were translated into English. And a lot of the rumors surrounding him was that he was basically a walking God incarnate who made everyone else look like, frankly, giant pussies.
I think a lot of the backlash against Broly stems from a combination of the above factors. Had he been a one-shot villain who decimated everyone and was only killed by what amounts to a
deux ex machina, he would have been a fine inclusion, just as a Bane who disappeared after destroying Batman or a Doomsday who stayed dead after his double-KO with Superman would have been. But subsequent appearances have been unkind to him, and because he was depowered, he makes the characters he dominated in Movie 8 seem depowered as well, and that doesn't set well with fans of those characters. And I have to admit that a lot of the core Broly fanbase seems unintelligent, to say the least.