I was once a vet tech and kind of know why he feels you should spay your pet. It helps prevent mammory tumors from developing later in life - for one thing. My sister in-law's dog ended up with this and we operated, but didn't get it all - too much tro get them all out and if you don't get all of it and the air gets to it (during the opening of the operation) - it spreds quickly and that is what happened. It was such a sad story - very nice dog and sad my sister in-law had to go through it.
2nd - it stops them from having their heats so no more bleeding.
And third he was probably thinking of ~ no accidents of her getting pregnant and getting a call - what do I do now!
Not to mention any faults the dog may have and would pass on to other pups or she may be too small to breed.
These are all just a few reasons to spay a dog. Yes, call him again and ask him to explain for you. This way you will know for sure his reason.
I would educate myself first. Read and research all about this yourself first and then
decide for yourself. A vet should not make that decisiion for you and yes he should have explained further why he felt this way. If you decide not to spay her - change vets and research and get references before going to a new vet. There is nothing harder than trying to find a vet that fits you and best for your babies. Remember - that vets are just people too.
Best of luck and let us know how it goes. We are all here for you!
T.