Now we've got spares our singer could keep a loaded spare in his pocket to change the whole compartment rather than just the batteries.
But that wasn't why we bought these packs. We bought them because we had to.
As in my review for the IEM kits, the IEM receiver packs are pretty solid except for one part - the clips break off the battery compartment really easily if the musician (or tech) drops the pack. And the musicians always drop the packs because they don't pay for them. Then the battery compartment falls out. Not good.
It isn't practical to apply tape to the clips before they break off as this slows down changing the batteries, but not taping the clips on means you soon don't have any clips and the entire pack needs to be taped anyway. Helpfully at that point the design of the packs renders taping the compartment to the pack intermittent.
So the existence of these compartments as spare parts is good, but the need for them is not good.