I obviously have the ability to brand them and it still makes very little sense to me. Sure, I can brand someone and have them become a Warchief. I can, if I want to, choose to help power them up...but I absolutely do not need to bother with that at all. The only reason to even do that is to have help with mostly easy (until 10,000 reinforcements spawn and I have to run away) Captain/Warchief fights.
The main use I have for branding is death threat. Sure, it's cool to have "your posse" do a surprise attack when you command them, but wholly unnecessary. The way the system was so majorly hyped up, I was expecting a real need to buff up your units, but there's not. If you've been doing the side missions, by the time you reach the second area you're pretty much unstoppable in combat. Quick critical chain, double specials (executions which also work on Captains/Warchiefs), area Wraith stun etc, just lets you demolish non alarm spawned swarms with ease.
As far as the power struggle missions; yes, you can kill both of them, but you can only kill the victor after the dispute was already resolved. The game awards you your miriam or P or whatever for what you just did (which I don't see the point of).