Unless you have a particular reaosn for seeking legal separation -- like a parent is ill and you want to be able to keep him/her on your health insurance -- I agree with the pp. There's no real point to it. If your intent to leave is serious, stringing your husband along is only going to make things bitter in the end.
To address your last point, many lawyers advise staying put till one of you can move out nicely, unless there's abuse. Everyone recognizes it's awful. But it's one of those suck-it-up things, unless you can find a way of doing it very carefully. Many states take fathers' rights seriously now, so I'll repeat, if you run off with your daughter, that may come back to haunt you in the custody determination,and make things awful for much, much longer. Get things straight with a lawyer first. Find one you like, and pay attention to the lawyer's local reputation.