Who else thought B.E.N. ruined the movie?

I think the problem was that the original story is pretty dark, and someone had realized that and tried very hard to make it more kids-friendly.
And so, not one, but TWO completely unnecessary silly comedic-relief sidekicks.
And so, a story in which all mistakes are instantly forgiven so that nobody has to die, and they all become friends instead.

He was indeed quite grating and one of the most grating Disney sidekicks, but I didn't thought he actually ruined the whole movie.

Because I know the original Treasure Island story I was kind of prepped for this character (named Ben Gunn in the book,) in terms of when he would come into things and the kind of roll he would play and as a result I have no issue with him. I don't love him, but I think it's a solid interpretation of the original (also out of his mind) character.

Now, had I not known the original story he probably would have annoyed me because without that context he feels unnecessary and just a way for Disney to force in one more goofy sidekick.

B.E.N being a slapstick comic relief character was redundant because they already did that with Silver's 'parrot' i.e. Morph, not to mention several of the pirates and to a lesser extent the doctor.

The whole idea behind B.E.N was pretty stupid anyway: why wouldn't Flint, a ruthless pirate, have simply destroyed or deactivated him instead of ripping out his memory core and letting him amble around? In the original story, Ben Gunn is only there because he had left Flint's company at some point and then returned to the island to find the treasure with another group, only to be marooned there by them out of spite for wasting their time when he failed to locate it.