I think we really ought to be asking why someone who finds Jackass to be comedy worthy of their time would honestly understand why Carlin is missed.
What did Heath Ledger actually do? Not "He would have...". What did he actually give his fans? A few movies? Carlin was a comedian renowned by many different crowds for many decades.
Even if his performance in The Dark Knight is stellar, it won't match. George Carlin being overrated is an opinion that doesn't really bother me, I may even agree...a bit, but the fact is...he has more material to be rated in ANY sense. Over, under or fairly rated. What does Ledger have? A Knight's Tale, Brokeback Mountain, Ned Kelly and...death?
-AC