Animal Companions and Barding (Proficiency)

[Dispari Scuro]

So someone in the same game I'm in is planning on doing a ranger with a small cat companion. They're discussing buying it armor, and I said armor proficiency is a fairly good feat. They said they don't need the feat, which made me wonder how the rules work in that situation and if there's anything I'm missing.

According to the rules, wearing armor you aren't proficient with just applies the ACP to your attack rolls. But plenty of light armor already has an ACP of 0, and you could wear up to MW studded leather or mithral chain shirt without taking any penalties on attack rolls.

If you're only going for light armor anyway, is the feat completely pointless? Is the only reason for cavaliers getting it so they can take medium or heavy proficiency? Can you throw heavy armor on any animal companion without proficiency as long as you don't plan on having it attack? Or is there some other restriction that applies here that I can't find in the rules?

[MrSin]

If you can hit 0 ACP there's no point in getting the proficiency, yes. Nothing is keeping, for example, a rogue from taking armor expert, and wearing a mithral breastplate better than another rogue without it wearing a regular chain shirt. This works out fine because proficiencies are rather weak feats anyway, imo.

[wraithstrike]

For animal companions it stacks with their natural armor pretty well. Getting mithral medium armor is not a bad way to go since the ACP will be low.