Like it's older uncle, the Freudian slip, the political gaffe is supposed to show us the truth beneath the mask we wear, whether that truth is repressed (Freud's real interest) or merely secret (the usual way we think of Freudian slips).
And perhaps we're particularly interested in political gaffes because politicians are such slick products: like iPods, politicians are not supposed to have anything but a surface. So any reminder of how politicians have invisible interiors will get our attention.
(Perhaps this also helps explain why charges of hypocrisy are always so interesting to us: Larry Craig's sexual misadventures reveal something interior and hidden, whereas a Barney Frank man-scandal would be a little less juicy since his homosexuality is on the surface. Although there we have an infidelity angle, and anti-gay advocates would make hay of that charge of hypocrisy.)
Taking a slightly different tack, On the Media recently did a segment on the life of political gaffes, which boils down to confirmation bias: if you think someone is an idiot, you're more likely to recall moments when they said something idiotic. I think this does a pretty good job explaining (away) many gaffes and their lack of power to sway: many gaffes expose a truth we're already supposed to know.
Which makes it seem like gaffes are really not that important when it comes to well-known, national figures, since 1) everyone pretty much knows what they think about these figures; and 2) these figures have a long history of speaking and acting that should reveal to us their priorities.
So, for instance, Perry says he's opposed to Social Security; and maybe his language is interesting (why "unconstitutional"?); and maybe now he's trying to walk back that language (or chew his way out--and as usual, cover-ups make you look worse than the initial problem); but if he hadn't said that, would anyone doubt that he's an orthodox Republican/radical conservative? (Is there really a difference now between those two?)
There are some gaffes that are revealing--for instance, since no one is supposed to be racist, a gaffe revealing a politician's racism may expose something previously unknown; but by and large, the political gaffe today is an exercise in high-fiving your friends. When was the last time a gaffe really pulled the mask down and showed us an interior truth?