Uh... mechs can presumably move sideways, crouch, crawl, lie down, and turn around much faster than other vehicles (useful for retreating.) Also, in a mech, you get a much better view. It is presumably highly modular, with easily replaceable parts. You could simply slap a handful of missile pods onto it (or anything you wanted), for various duties. Yet another thing is, you could make use of the mech's modularity to have it firing a bunch of missiles one moment, then simply swap parts for salvage, repair, and building duties the next. In battle, the mech could crawl forward, then lie down in a favorable spot, firing missiles all the while. Mechs, properly done, seem pretty useful to me. When standing, they would make great radio towers, too!