The thing LAMs bring like nothing else is operational mobility. The ability to move a lance of 'mechs around at aerospace speeds without a dropship is amazing. As others will point out, for actual fighting, you want to be in 'mech mode (or AirMech mode if you're transiting at something less than ASF speed). Duking it out 1 to 1 with actual 'mechs or ASFs will land you in a heap of trouble in short order. LAMs are for smashing support forces and depots flat, then heading for the hills before the dangerous units can respond.