I think there are a couple of issues going on here.
Let me explain. I prefer my roleplaying in very small doses. I don't enjoy having half a session devoted to talking "in character", nor do I like having to speak in first person for my character. I like combat, I like tactical maneuvers, and I like killing things and taking their stuff
I've been playing and running games for twenty-six years now, and I really don't enjoy the funny voice thing either. One of my friends describes the way I handle NPC interactions as "meta" because I don't speak first person unless every word they say is important, or we're dealing with very short responses. The PCs, in-turn, respond in a similar way. Not everyone is interested in being an amateur thespian, but you can still have a full range of interactions without having to get into the exact wording or phrasing. When I do quote their exact words, I put it in quotes, as you would see in a novel so that there's no confusion between my words and a character's words.
There's nothing wrong with this approach and you can have a perfectly satisfying game that way. On the other hand, if you like to RP in first person every encounter, going so far as to do the haggling with a merchant over a non-magical mundane item, that's cool too - but it wouldn't really fly in my group. Other groups will vary.
Roleplaying, to me, is something that should be the background and not the major focus. I don't want to "play stupid" with my character, and therefore make poor tactical decisions that hurt the rest of the group. Maybe it's because during the past year I stopped playing D&D and started playing WoW, but it's now my view that someone who hurts their party for "roleplay reasons" is a total jerk and doesn't belong in the group.
This, I think, can be an issue. It really depends on what you mean by playing stupid. Roleplaying was originally conceived as a means of simulationist immersion in a fantasy world. Sure, it has evolved to mean different things to different people, even varying by the particular game you're playing, but the one rule I do keep at the table is that there needs to be a wall between player knowledge and character knowledge. Just because you know the Achilles heel of a monster doesn't meant that your character knows it, and I frown upon players ignoring that fact in my games. Granted, sometimes it is a bit frustrating to pull out that killer move right when you need it, but not doing so makes the game function on a more authentic level, or at least that's the way I see it.