I play a triform spec PB in pvp one hell of alot on virtue (sirens call mainly because rv has waaaay to many groups of mobs) And no u don't really wanna spec for shields in trispec u are ether using heals or dwarf form to survive, i only use human form for IS, finding low stealthed stalkers n such and knocking ppl who don't know what hover is out of the air. (brutes jetpacking towards u is always fun, lead them up to the top of the map annndd BAM :P)
As mentioned, the shields would be a waste of time, slots and power choices. If you are going for a tri-form build (my preference), then you'll be shifting far too often to get use out of the shields.
Will shields be a draw back for a tri form in PVP or will it add to the pb's survivability? Since keybinds can only activate one power at a time I see the shields as a drawback if you are shifting in the heat of battle. You really can't take much time trying to get shields up.