First, you're assuming I'm saying he couldn't be used for roleplaying. I'm not. So there isn't any argument of mine to counter in that regard. I'm saying that before anyone says he's too powerful, or strong, or however you want to put it, look at the type of campaign that is being run. If it's mostly roleplaying, or otherwise non-combat related - there are plenty of non-combat, non-roleplaying aspects to the game, such as tracking, or climbing, or whatever - then he's not too powerful. In fact, he's probably not going to be on par with other characters. Yes, just like the fighter, in many instances.
We have seen any number of threads throughout the life of EN World that equate power with fighting ability. Classes like the ranger and the paladin have been proclaimed weak because they don't match the fighter in the ability to deal damage. In campaigns that are combat-heavy, then maybe they are too weak.
My point is that the character we're discussing here is, indeed, a glass cannon (or an egg with a hammer). I can think of any number of ways he can be rendered relatively powerless in combat, as have many others - and taking him out of combat makes him almost powerless. He does one thing, and does it well. And that's pretty much it. Take him, or classes like the fighter, out of their element, and they aren't particularly noteworthy.
One can roleplay however one wants, but the player characters that put ranks into social skills and have abilities that relate to aspects of the campaign other than combat should get just as much advantage from their strengths when in their element as the character above gets in his element. To do otherwise would penalize players who have created characters that are focused on things other than combat, and would establish that combat prowess is, indeed, the only real indicator of power in the campaign.