It's a lot easier than most people realize to leg a mech. You can torso twist if you are about to be cored, or just to spread damage preemptively. But what do you do if someone faster than you is trying to leg you? You can't twist your legs away, not fast enough like you can your torso anyways. Legs can be shot from the front, side and rear. I see people in this forum say: you don't need that much leg armor, shave some off. I've seen Atlas builds linked with leg armor in the upper 40s, mediums with less leg armor than lights. You may say, but I have 2 legs and only 1 CT. That's true but taking out both your legs becomes much easier once you lose one. Not to mention you are at a major disadvantage even if you do survive with your one leg. Also, there goes the ammo you probably stuck in your leg, and possible ammo explosions. Don't shave leg armor so much!
TLDR:
It's easy to leg people right now, put armor back on your legs.
---EDIT---
Here's an anklebiting compilation:
Edited by Indoorsman, 31 March 2013 - 02:38 PM.