Near as I can tell there's no point--in fact it's probably more worthwhile to use them up as much as possible to let your personal brigades get better shots and more xp. Them dying might even give you back more equipment from returned. Needs testing to see if having higher intact free troops gives you most post-battle.
In mine I kept the two best infantry brigades and both artillery brigades. I suspect it will automatically give you the higher 4 XP brigades, but again needs more testing.
Hitorishizuka's post in Preserving allied troops was marked as the answer