In most competitive battles, I try not to use legendaries mainly because most of them are considered Uber. I like the Overused tier, so I usually stick with pokemon from there. The only pokemon that's close to a legendary that I've used in competitive Overused battles is Celebi. I do used legendaries in the Uber tier though.
If I'm going through the pokemon storyline, I don't use legendaries period. Reasons as to why? I think it's just a force of habit. I started at Pokemon Silver and I wasn't fond of the legendaries there. So I didn't use them and that became the start of my habit I guess. Any other reasons would probably be my laziness. I mean, I already have high level pokemon that are probably higher than the legendary, so I'm too lazy to train yet another one to get it up to the average level of my party.