Only if you want to go riding.

The Bedouins used to prize their mare's milk, but apparently that's not a done thing in Skyrim.

I left out the milk cupboard in my kitchen, and I never have my churn respawn since if it did it would mean all the junk in the house would respawn. I have never had any trouble getting plenty of milk and butter from vendors and have pretty good luck with Marise Aravel who has sometimes had 2 or 3 butters at once.