It'll just fill it up.
Issue 16 made me feel like this.
Warning: This poster likes to play Devil's Advocate.
Note that you can switch your Brute to Rogue or even go all the way to Hero, and then come back to Villain, and recieve the power again.
Also, you can only have 1 Alignment Temp Power at a time. So, if you have Frenzy, then become a Rogue, you loose Frenzy, and get Countdown to [Rogue Temp Power] (I forgot the name of it... 
).
Basically, you're allowed to side switch, and still get the power. You just have to run 10 Tip Missions, choosing the Villain option, run your Villain Morality Mission, and then spend 7 days with your Alignment set to Villain.
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Biggest Troll on the forums? I'll give you a hint:
*Possible (minor) Spoilers Below*
Read on if you have already completed the villain morality mission and/or do not care about (minor) spoilers.
Am I to understand that my pure-Villain Brute will have access to a power that will fill his Fury bar and provide +60% recharge for 20 seconds? Will this make my Fury immune to decay for the full 20 seconds?
If I read something incorrectly, please tell me. Otherwise, I am very excited.