Mechs are cool, they carry alot of weapons, and it looks cool when they attack.
But one problem, and I think one of the biggest reasons why you don't build them. As with tanks your silouet is very important. The closer you can be to the ground, the more things you can hide behind, and the more combat effective you will be cause you can deliver your round down range at your target without being as exposed to incoming fire.
So you have this big three story mech, or something like that. What does it say? Hit me, big, load, target right here. Shields, and SIFs can't do anything if it lumbers around and is vanuerable against Phasers, disruptors, photon torpedoes what have you.
That's why I wouldn't use them. Sorry to kill the idea, but someone had to be the devils advocate in here.
But mini-mechs, or body suit mechs would work well. Enhancing the fighting ability of the soldier. Kinda like Exo Squad if you all remember that. Or those small personal mechs like in the Battletech series that use to run on UPN here in the States.