I've always had an aversion to shields on mechs. Why not just make the whole frame with a strong alloy instead? Tanks, IFVs, etc. are given armor in case they get hit, but best practice for all assets is simply not to get hit on the first place. This is especially true of FRAMEs; combat doctrine emphasises maneuvers and use of cover that reduces the chances of getting hit. 

Any weapon that can penetrate armor can penetrate a shield as well, damaging the arm and making the machine that much more combat ineffective, or worse, piercing through the shield and contacting the center of mass.