Ahhh, bar ends. Well, personally, I like bar ends. I use a pair of old Specialized that I bought about ten years ago. They're "L" shaped. One of the criticisms is that bar ends catch on branches and stuff to whip your handlebars around and throw you. Yeah, maybe that happens with straight ones, but I've never experienced it. That's why I use the ones that have a bend in them. The way I figure, if something gets me there, I'm prolly going to eat dirt anyway. (Now, there was that f*cking tree what grabbed my helmet in Kernville and yanked my donkey off the bike. That was fun.)
I use the bar ends when I'm climbing. They put a little extra weight/leverage over your front tire on steep or technical climbs to help keep the nose down so you retain control. And, like EMB, I use them on long rides to change hand positions. Sometimes, in using them, I feel more stretched out in the cockpit and have a little more pedaling power for climbing, too. Strictly speaking that's what they're for; climbing. You never ever want to use them when you might need the brakes. Unless you get all emo and want to die. Then, latch on and ride the black wind, baby.
The "L" shape also protects my hands to some degree, actually warding off branches and stuff. I attached a picture where you can see the bar ends (if you squint and sh1t 'cause it was a bigger pic on my computer). I position them so that they are mostly extending forward from the handlebars but slightly up so that they are a little above my knuckles.
The other thing is that they look cool. I don't give two sh*ts and a squirt what anyone says. My bar ends look tight. They're like devil-horns or some sh*t. (And, as soon as my gut melts away, I'm buying lycra, too, b*tches! Aww yeah!) And, as you know by now, the most important aspect in mountain biking is looking cool. That's why we wear such bitchin' helmets, y'know.