They've said before that companions have their own strengths and weaknesses, and wield certain weapons more effectively than others. Veronica is mainly unarmed, who's default weapon is a power fist, Boone is a long range sniper, so he's obviously better with using scoped rifles. Raul uses revolvers, Cass uses shotguns and rifles, Arcade uses energy weapons, and Lily uses melee weapons (as far as I can remember).
So it wouldn't make much sense to give, say, a minigun to Raul, a plasma caster to Boone, and a flamer to Veronica. Instead, you'd try and equip them with weapons they'd wield more effectively, like giving Veronica unique power/ballistic fists, giving Raul a better revolver, Boone better rifles, etc. etc.
Of course, it probably wouldn't hurt to give them a backup weapon, in case they end up running out of ammo, or what they're using just isn't effective.