Really, in the movie they never should have introduced the idea that "curing" Ben was within reach simply because of the reasons already stated, it means the character pretty much HAS TO choose to remain in his current state, even if it defies logic. You might as well have had Ben say, "No thanks, Rick. I know you can cure me but doing so would render my character useless and we're trying to start a franchise here."
It's a subject they shouldn't have even broached and just left their conditions as being incurable. It's been a while since I've even seen the first movie, what was the reason Ben was shortly turned back human? Dr. Doom needed to use the machine to make himself more powerful... or something?
Anyway, it's a topic that really shouldn't have been breached or suggested because it simply opens that door for people to say, "Why isn't he cured yet? Why not switch back and forth?" It's sort of like in every other episode of Voyager they came across a way to get home but by the end of the episode couldn't because then the show would be over. It's Gilligan's Island Syndrome.
Hell, if they really "wanted it both ways" go the way that 1990s, never released, movie went and give Ben the "ability" to switch back and forth.
If you've not seen it: In the never released movie from the 1990s (IIRC only made so the studio could keep the rights to the movie franchise. They simply had to "make a movie" even if they didn't release one. So they threw one together for like a million dollars.) A couple of times in the movie Thing reverts back to his human form, usually when he "let his guard down" and let people in. In particular around Alicia Masters. (Going along with the idea the F4's powers manifested from personality faults with the group.)
Granted even that sort of mutes the "tragedy" of the character and makes him that much more like the Hulk but it's better than, "Meh. I'm fine being a hideous, giant, monster who can't go anywhere or do anything without breaking things, causing great damage, or being a big hindrance to everyone and everything. Hell, I can't even sleep with my girlfriend without destroying her vagina. So, leave me as giant rock even though we can totally fix that right now."
But, again, I think Chiklis did a great job as Grimm/The Thing as is probably the best thing about that movie short of Jessica Alba's scantly clad body. I didn't much care for Ion Guerillaglue as Reed and Evans actually was pretty good as The Human Torch, he played "cocky asshole" pretty well. The effects used for THT weren't very good, though. Looked nothing like fire in a time when CGI Fire was a 101 class in "learn to be a CGI animator school."