Depends on your budget and how much time you have when you're swapping wheels.
I had 2 sets of wheels for my MTB, I had the second set built up with the same rim as what I already had so there was no need to adjust the brakes.
If you are putting a new casette on your second set, its generally a good idea to put a fresh one on your current wheels and replace the chain, it'll help with shifting and will run quieter.
It is handy to have 2 sets, I used one for commuting and one for trail riding.