Well, my phone is being a butt, so sorry if this posts twice.
We all have different feelings in this regard, but I will try to explain things a little more succinctly.
When I originally began breeding Sphynx, I would allow new Moms and Dads to come over as soon as the kittens received their first set of vaccines. This continued until one day, I had a new Mom call me extremely upset to inform me that the same day she came over to visit her baby, she also had cuddled and handled a puppy coated in ringworm. Now, she didn't know the puppy had ringworm at that time- and she couldn't recall whether or not she handled the puppy before or after visiting my cattery!
No biggie, right? Um, wrong. Thankfully, she must have seen the puppy after her visit because no one came down with the dreaded fungi, BUT it could have easily happened and all by accident. If those kittens had come down with ringworm- some owners probably would have backed out of their purchases and others would have been left waiting to receive their kittens far past the promised delivery time.
The fact is- these kittens are MY responsibility, but I also consider the future homes that already have put down a deposit as well. THEIR future happiness depends upon MY responsibility within my cattery with THEIR kitten. To me, it's not worth the extra risk of exposing my kittens to more outside hazards for the off chance that I'll sell an extra kitten to someone down the road.
As most breeders who have done this for more then a litter or two, I've had the horrible responsibility of calling an owner and telling them something was wrong with their kitten (Neala- with her Persistent Right Aortic Arch- I had to call Amanda and tell her that something was seriously wrong with her baby). I've had the horror of having to call an owner when their kitten died as well as to ask permission to put their baby to sleep (Tiki and Bamboo- both developed extreme reactions to their vaccines. Tiki died that night in my hands and Bamboo, I put to sleep the next morning after confirming my actions with his future Momma- I didn't have to ask her permission, but I felt in that situation it was the appropriate course of action). You already have such fragile little lives in your hands from the moment they are born- it is up to YOU what risks you are willing to take with them.
For some, the risk of having visitors is minimal- or they view it as, the kittens are exposed to germs day in and day out, so why worry? (For example- I do have kids, so I know the risk of walking in something like ringworm is always a possibility). For me- I am a born worrier and I will avoid risks that are unnecessary. I cannot kick my children out of the house, but I can refuse admittance to strangers who have no purpose within my cattery.
Once again, it's all about point of view. To me- if a potential owner cannot respect my need to protect my kittens and the future happiness of homes who have already committed to a kitten, then they don't need one of MY kittens. There will always be someone else who will sell to you. It's not a loss to me.