One of the suggested uses for the interchangeable puck I've seen online is that you can buy a premium band once, and then put a fresh puck in it from your free resort MB instead of buying an entire replacement if the battery on the premium band dies. Other than that, yeah, it's for letting you put it in a fob (there are official ones as well as some cute stuff on Etsy) or mix and match colors.
To the OP's question: I'm never 100% confident the battery is live on my MB even if it's a new one--I don't know how long it was sitting on a shelf before they sent it to me. If you wear an old band you might not get the magical effects on some rides (It's a Small World will sometimes show your name in the goodbyes, etc., if it can sense the active signal from your band), but you can still get your ride photos even with a "dead" MagicBand. When we went in 2015 (our first trip after the system was implemented) we made a point of tapping our bands at the ride photo view screens if there was a photo we wanted to attach to our accounts. Even if you have a band with a working battery it can still have errors matching your photos to your account, so I figured it was worth doing if it was a photo I cared about. I plan to bring my old band from 2018 even though I expect the battery to be dead, because I want a second color option. Probably not the 2015 one since it's 1.0 and not as comfortable to wear, though.