We came across this issue with our vet a while back when Ayasha was about 4 months or so. The vet kept pushing to have her spayed at 6 months and cited the typical reasons vets do for spaying this early. She was worried about Ayasha's eventual size and the drugs it would take to knock her out at an older age. Our breeder and us have a different plan for Ayasha than our vet. We are waiting until she is much older, about 18 months to 2 years of age. We know this means going through a cycle, but we are much more comfortable with our decision based on our breeder's expertise as well as the threads I have read on here.
In the end, it is your call and not your vets. I had to put my foot down a bit with our vet and explain that while we appreciated her knowledge, based on our discussions with our breeder and their extensive knowledge of the breed we are waiting. At least she has let it go and is not persisting.