Updating an app doesn't require a lot of battery power unless you have like 20 apps that need updating at once. The limit of the update is still 100MB over cellular so it's not like you're going to update a 1.2GB game that would definately use a lot of battery.
I agree. This drives me nuts! I don't need to recharge my iPad but about once a month and would love to update apps without having to plug the iPad in (I don't even keep my charger that handy since I use it so rarely)