That doesn't hold true for my phone at all. I can install Franco's kernel or updates to it on a existing AOKP build with no battery life issues and have many times already.
Just adding in my experience.
I didn't have any bad experiences but I accidentally deleted my nandroids and was forced to do a fresh install and I've gotten a few less wake locks since then.
No need to wipe if everything is good, just a what to do for those experiencing problems with franco 12 and aokp or bixies for that matter.
Sent from my Google phone