Some mechs just have to be legged to counter them *cough Owens B cough*, but I can say the I rarely leg even Scats, as it is easier to just go for a torso or weapon.
Legging simply isn't a problem for bigger mechs, I will say though that the current physics system of face planting below the ground geometry makes getting legged really REALLY annoying because you can't shoot back or anything.
Believe me, there have been long discussions of how the legging system is and if it should be changed, if I recall the poll ended up being the leg one leg get limp, leg both get fall, but I also recall that there was enough opposition to that fix that it remains inconclusive on what to do with it. Thus there are no changes I'm aware of planned, who knows what the devs will do though.