The question then becomes "how much access are you giving to the companions?" in an MMORPG. I LOVE the XI Trust system, but that comes from the want for solo content. The only reason it exists is because of that lack of players to form parties. Giving players a means to bypass a need to interact and deal with other players (because of lack of a necessity or to avoid the bad ones) has a noticeable history of removing group needs. The Trust system is an obvious one, but another is something like Garrisons in the current expansion of WoW. While it doesn't involve NPCs joining in you combat (technically), it does involve removing the player from needing groups to get what you want. (Edit: This is referring to the perspective that players willingness to group declines further, not that the group content won't be there.)
While this is obviously not the case for everyone, and it's clear they wouldn't allow such a feature to work in max level content, it would absolutely hit the mentality of many players to avoid groups or interaction whenever possible. If players are okay with a move that creates different content away from groups/interacting (think if PotD was solo only with an NPC party if wanted), then it'd be fine. I'm certainly okay with it (since it'd essentially make the game more single player friendly with varied content), but I'd imagine that sooner or later, others wouldn't be lol.
My assumption was that it'd be for primarily low level content or select story dungeons. Stuff that really doesn't matter on the mechanics end that couldn't just be brute forced through. That would obviously mean that the few mechanics that are dangerous to us, the players, would have either have an improved AI or not affect NPCs as much (like if the companions had a natural buff that reduces environment/AoE damage taken by 80%). Notably, this is referring to bosses like the last one in Stone Vigil or maybe the ones in AV.