it depends on YOUR daycare. If you have a daycare open year round, then I'd say if they want a spot, why should you be out $$$ and work around their schedule. But, if you WANT that time off, or reduced work load (having less kids) then it's your choice to cater to that, as you're both getting something out of it.
Personally, I'm like dreamalittledream and ONLY take teacher's kids. I"m closed all the same time's as them ,but don't charge. It's my choice to work that way. My husband is also a teacher, and neither of us want a house full of kids when he's off! But , again, it's my choice, and I advertise as a daycare that's closed all summer, every summer...