The Posse are kind of fragile and weak individually. The idea is that numbers can make up for any faults they may have, but that stops mattering at higher levels. They simply need to be buffed by your secondary if you don't want the whole group to be oneshot by the first aoe to hit (When applicable. If you're debuffing, this obviously doesn't matter.). Combine this with the fact that you can only summon them in the heat of battle and you'll be lucky if more than a couple survive their first brawl.
So turn the power into a ground target and allow us to summon and buff the gang at our convenience. This is doubly useful for Pain and Time, where the big buff is on a long cooldown.
