I could see shields functioning primarily to mitigate the effects of shrapnel and indirect ordnance. They could absorb the force of explosions nearby the FRAME and prevent flying debris from obstructing the operation of the machine. They probably wouldn't hold up to a direct AP autocannon round, or anything larger; anything designed specifically to penetrate armor could put a hole in a shield, and the damage could very well just pass onto the body of the FRAME itself. Glancing hits might be mitigated, and shields could provide a buffer against HE grenades, mortars and small arms fire that could hit an optic or other vital piece of kit. Shields would probably be more useful against infantry, and I imagine police FRAMEs using them in anti-riot contexts.