I'm a huge fan of self-effacing humor. Like most things, it can be amazing when applied sparingly and in the right situations. It's disarming and often endearing, but only when it's authentic. By 'authentic' I mean someone poking fun of themself for something they actually are -- fat, short, bald, single, broke, disabled, uncoordinated, whatever.
And by 'sparingly' I mean once in a while.
But when self-effacing humor is just lathered on like it's going out of style, I find it very annoying. Here's why:
(1) It's insincere. I believe that deep down, everyone has an endless reservoir of faith in hir or her own abilities. We have different ways of showing it, but we all believe in our unique talents and will go to any lengths to rationalize about it (Just think, how many times have you heard, "So-and-so may be book-smart, but I'm street-smart.") So when someone makes a reference to himself being a caveman, a monkey, or a knuckle-dragger every five minutes, the insincerity starts to grate, quickly. No one really thinks that about himself.
(2) It's awkward for everyone else. Again, when self-effacing comments are just poured on with no end ("Leadership? All I've ever led was a drunk to the bar!") all you can respond with after a while is a groan. Not only is it not funny, but it makes you wonder -- is the person saying that just waiting for you to challenge it?
(3) Most important of all, it shows no 'ownership' of a situation. For instance, let's say you were trying to teach me how to drive a stick-shift. Your goal is for me to learn to do it on my own (that way, you can end the lesson and go home!) Well, if every two minutes I just say, "I don't know, I'm a caveman" instead of just taking ownership and responsibility for learning, that's actually unfair to you. One or two jokes about being uncoordinated is of course fine (refer to earlier paragraphs) but endless self-effacing comments aren't going to help anyone in the situation. It's especially not cool when the person doing it is the one in charge.
Here's the bottom-line: self-effacing humor is one of the greatest social skills you can have in your arsenal. It can quickly beat the awkwardness out of any strange situtation. It can instantly disarm people who've formed contrary opinions about you before they've even met you. It can be a great way to break the ice in front of a small crowd at the very beginning of a speech.
But like all great things, it has its place. Don't abuse it.