Clare, your points are valid but I will tell you, if I can't come to somebody's place and look at their conditions and birds, I wouldn't buy from them. I don't care who they are. As MCM said above, one can sound like a real know-it-all on the phone but when you see one's set up, facilities, and birds themselves, that can tell you a whole lot more. I've been to quite a few "breeders" places and I can tell you one look at filthy conditions, waterers that look like they've never in their life been cleaned, birds running around with legs swollen with scaly leg mites, etc. sent me looking elsewhere. There are lots of other reasons to personally visit one's farm but anyone can go online or read books and find out all that stuff.
As for Disease - it's not all that hard to take good, effective, and simple precautionary measures. Mainly, ask visitors to not wear their "chicken clothes" when they come and if they come with their work, rubber or other boots that they wear in their chicken yard, just do a good oxine or chlorine foot wash. Moreover, I would submit that one is just as likely to pick up a disease at a show (which isn't all that likely but with the gaining popularity in backyard poultry is becoming increasingly more so because of innocent beginners not knowing better) as it is from someone serious about getting good breeder birds coming to your place.
Asking good questions - and a LOT of them - is definitely something one should be doing. But it can never take the place of seeing the breeders place with your own two eyes.
P.S. I encourage people to come to my place. I want to visit with them, answer their questions, and begin to build a relationship with them. It'll pay off in the future.
God Bless,