MKX said:
I think you are reacting to a possible situation that Fantasy Flight does not intend to happen with these rules. From the latest developer diary, it seems like Grey Knights are intended to be in an all Grey Knight squad. I don't think they honestly intend for one of them to be included in a group of acolytes on an investigation, or a hit squad for that matter.
I agree that including them in an Acolyte cell in a completely inappropriate idea. If for no other reason than the fact that there is no reason for it to happen in the first place. Keep in mind that unless there is a daemonic incursion of some sort, the Grey Knights don't typically get called in.
As an aside, there are plenty of things that can ruin a space marine's day. The ever popular Genestealers, for instance, will slice them into ribbons. They also don't stand up to some heavy weapons very well, plasma and las-cannons being foremost on that list. Can't really imagine them escaping a hit from a power fist without at least some damage. Pretty sure there are some psyker powers that can prove nasty, as well.