Exactly! Most nightlies don't contain anything for the Droid X. The only reason I flash to the latest nightly is I get bored.

The whole belief that battery is better on Nightly #7 is false. Nightlies are development snapshots. Every night around the same time, one will be uploaded regardless if changes are committed or not. It's an automated thing and will happen regardless if someone is actually committing changes.
Like you said, there have not been any changes dealing with battery. Well, the 1% increment thing was a partial commit and doesn't actually change anything yet. It just laid the framework for a later update. Nothing between #7 and #16 should affect battery life whatsoever. Any changes are because of "placebo."
Cvpcs made a great article regarding nightlies. I'm too lazy to provide the link right now, but anyone can stroll on over to the Cyanogenmod forums and look at the first post in the nightly thread for the Droid X. His article explains exactly what nightlies are and eliminates any misconceptions.