When all your devices memory is filled with bloat that you never use, and the apps you always use constantly redraw that is a performance issue, and it effects the battery (not much, but it does). When you then freeze or uninstall the bloat and your other apps no longer redraw constantly while you use them you have increased the performance. It's not as much of a problem now, but with older devices with under a gb of ram it was a major problem. I also understand that apps sitting in memory don't (normally) eat up battery or resources, but when its bloat and your other useful apps are never in memory it's a problem, also its a problem when your device starts to lag due to low memory (was only a problem for me on the Droid X and Thunderbolt, every device with 1gb of ram or more has been fine) I had to use a app to reset the ram settings so it wouldn't drop below 90 mb, or it would lag/freeze.
Backgrounded processes aren't using resources though. They might be in ram, but they are cleared out as needed, and that model works really well.
You use the same amount of battery whether your ram is full or empty. Empty is useless, and full is fine as long as the memory management works, and it does. You would only get redraw if those processes weren't backgrounded, and if they weren't, let me tell you, there would be tremendous battery drain. We would see screenshots of bloat taking up battery usage too.
Debloating is useful for:
1. Saving space on /system and probably internal storage.
2. You don't have to look at those crappy apps you don't use or want in the drawer.
3. You are OCD and have to keep a tight ship.
I fall under number 3 mostly, but I have no preconceptions about it affecting battery life or performance.
