I personally don't leg.
HOWEVER, I do see the merit in it. Most people think "Psh, eat the torso, you get points and its faster."
Well, if you engage and destroy the mech's legs, then the pilot is sitting there unable to aid the fight. Most importantly, she has a longer time to get back in the fight than if I outright gibed him. She either has to get out and poke someone as battle armor till they kill him, or walk back to base, or suicide. All are quite nice, seeing how you took a fight of maybe 6v6 on a flank, to 6v5 for much longer, or he suicides and doesn't progress in rank and must fight with weaker assets. People fail to see this.
And about the hiding the leg thing, when I did leg, I forgo the hip for the large sized feet and cankles. I know exactly where they are going to be, why wouldn't I shoot them? You can't hide them, unless we are talking about a snipe fest. Any terrain used in a brawl is a liability, since the ability to circumvent the terrain is very ready, and often only gains seconds of protection. Seconds against a burst weapon are mostly meaningless, especially if it will only take a single shot.