I agree with Alan here. A unit in skirmish or open order will benefit from the +1 save and will also suffer only being able to fire a single shot, but I don't see why they should suddenly improve their shooting skills just because they've got a bit more room than they usually have.
Actually I am not sure that I read old_serge's post correctly the first time
With respect to Mixed Formation the flank companies doing the skirmishing, are the elite companies, that are likely to shoot better than the centre companies. So I would give them the +1 on their single Die.
So in summary:
Line unit when in open order - no bonus Line unit in Mixed - +1 bonus Unit with Skirmish ability - +1 bonus