Role-playing rules are a delicate flower.
As others have mentioned, books that cram in a lot of fluff are often scorned by their target audience, because we don't want THAT fluff. We want our own fluff.
DMG2 is an interesting example.
All the stuff it had on fantasy medieval society & law: Great. Useful. Loved it.
All the stuff it had on Saltmarsh...: Skipped it. Somewhat interesting, but not that useful.
I don't know enough role-players, but I assume (perhaps incorrectly) that the way I reacted to the DMG2 content is the way most others do. I can easily imagine a scenario where I would use Saltmarsh, but if I'm a role-playing guy I'm probably using my own stuff, and if I'm not a role-playing guy, I'm probably not even going to bother pulling that premade scenario off the shelf.
"The mechanics of the system influence the way the game is played."
One of my players convinced me that this is true.
Yes, people who role-play are going to role-play and people who don't won't. BUT for those who DO, the role-play will tend to be richer if the system itself encourages it on some level.
There were all those arguments on 3e killing roleplaying and whatnot. While I don't believe that's true, I do think that the very attractive GAME elements, strategic elements, tactical elements, combined with the wealth of options for tricking out your character mechanically which were available in 3.x did tend to lead to players spending more thought in that direction.
For example, if I come up with a super awesome role-playing concept about my character's dark and secret history, his noble birth, his former lover who is now his arch enemy, his awesome swordsmanship... and then I assign my stats and feats to match what I feel that character's Charisma might be... maybe drop his Constitution to explain his sickly childhood, etc...
And then ANOTHER player in my group approaches his character from the opposite direction, choosing the very best mechanical combos for his character, well-- his character is going to be far more awesome than mine. Because when we're playing the actual game part of D&D, his character is going to be the one kicking butt. Mine is going to be the one falling down and failing his Fort saves. He's only super awesome in my imagination.
I dunno. I think the deal with 1e and 2e were that you didn't get as many cool abilities, so a lot of times the role-play was more prevalent but the GAME wasn't nearly as good (don't get me wrong, though: min-maxers were huge even then). In 3e, the mechanics are very strong and the GAME elements are a lot more solid, so it's easy to be attracted to that.
I'm very excited about the social interaction things they're talking about in 4e, though. If role-play is built into the mechanics somehow, that may be enough to encourage it. Right now, it's very subjective so it can't be "ruled." I understand that White Wolf has built role-play into the mechanics of their games, so I'm sure it's possible.