1. I believe we gonna see same for premade bg's as we see now for icc pugs in trade. There are the ones that just going to fill the premade with bodies, and there are going to be the coordinated ones. I take it you want a spot in the latter.
2. Much will depend on you. If you know your limitations and if you prove to be an asset for the team, a multiboxer will be def. wanted by many premades. For example: if you expect to fill 5 slots in a serious premade WsG, I doubt many multiboxers will be a real asset. But if you box your pally tank + disc priest and resto drui, you will make a very strong flag carrier, and thus wanted. If you run with your tank + 3 dps + 1 healer in AV, it means you can rush forward at pretty much any point to a pve boss and kill it = no problem to get a spot imo.

Myself I'll adapt my setup to each BG and bring the best comp possible as a boxer, be it running 2 or 5 toons.