Okay, three or four instances of Williams doing good work in the last 15 years. We also
Night at the Museum 2 (the first one was okay)
Man of the Year
Patch Adams (melodramatic schlock about a quack doctor)
Death to Smoochy
License to Wed
Looking at his IMDB page those are the ones that stuck out the most to me as being his "bad" or "questionable" work where he pretty much just acts like a coked-out idiot the whole time.
done some good dramatic work (One Hour Photo, Dead Poet's Society) but from the looks of things most of his work the last couple decades has either been forgettable dramatic pieces (which can go either way if they're movies no one is likely to have remembered), "act like a lunatic comedies", "odd situation comedies" (like Ms. Doubtfire which while... "okay" still was only funny because it was "Robin Williams as an old woman.") or genuinely, good, roles. Bicentennial Man I could go either way on.
He's funny, has (had) some good stand-up and done some good even great work. But he's really a mixed bag and I think he's another one of those actors whom the director has to "control" to get good work out of. But maybe it's just me but when I look at Robin Williams I don't think of "hard acting, dramatic ball of awesome" I think of hirsute coke-addled nutcase.
Maybe Nolan could get something "good" out of him but, personally, I don't think he'd be a good fit.
All IMHO, of course.