No, I also would not leave a bitch after a spay, never needed to ever, it should be a straight forward operation and most dogs walk out of the vets once fully awake. Unless a dog is out sparko most dogs can't wait to be home they don't understand why they are there,
Also as pointed out many vets try to get an over-night stay, I got talked into one once and then found out they did not have night staff so animals are left completely alone anyway,

what good is that? The argument that they are more likely to get a peaceful nights sleep and rest doesn't wash with me, I'd rather be there, IMO it causes more stress and worry for a dog. If your vet does stay all night that is good but not sure why he needs observation for a spay when they are one of the most common ops around.
I just sleep downstairs with any dogs now who have had an op and they sleep like babies in their own home with their loving owner, also if the worst happens and something goes wrong, you are there to call an emergency vet as you are giving one to one and may even spot something quicker than an overnight stay even would, and if something goes bad I'd rather have my dog in my arms than not be there at all.
Make your own decision on this........ but that is the way I feel about it.