I already have great battery life, so I never considered turning Apple Intelligence off. I'm not on my phone all the time and can go three or four days between charges, which has been the norm for me.
It's not like Apple Intelligence is constantly running in the background and consuming the battery. It's only going to do stuff when you're doing stuff. If you're generating a lot of images and emojis, using Siri / ChatGPT, or playing around with the other AI features, then yeah...the battery is going to drain faster. By turning it off, you're just forcing yourself not to do those things, and therefore saving battery that way.
Say you like to play a game that eats up battery power while you're playing it. You don't uninstall the game every time you're done, do you? Probably not. Simply closing the game is enough. I think the same can be said for AI. It's only going to consume battery when you're actively using it.