Let's compare shall we?
Now, take a heracross. It's base stats are:
Hp-80
Attack-125
Defence-75
Special Attack-40
Special Defence-95
Speed-85
Not bad, right? (Base stats are approximately what the pokemon's stats are when it is at level 40, kinda.)
Now, let's take at one of the well known Legendaries. Mewtwo:
Hp-106
Att-110
Def-90
Sp. Att.-154
Sp. Def.-90
Speed-130
Heracross was killed in almost every category. It was lucky it won in the attack stat. So you see, I can painstakingly raise a heracross, giving it the best care I can, and in one move, I could get wiped out by a mewtwo that somebody just caught. Do you see the problem? If everybody used legendaries, their opponents would have to use legendaries against them to stand a chance, making every other pokemon out of the available 386 worthless, which doesn't make a very fun game. And that's why I don't like legendaries.