A friend of mine and I were talking along these lines just yesterday.
It makes a big difference in how you work with horses if you are what I call a OHW, or one horse wonder. That person will not have the background when all they have ever dealt with is a couple of horses, of their own, that someone will that has worked in some capacity in a training barn, or on a ranch that uses horses.
Working with 20 or more horses daily, gives you a whole other look at how horses behave, think and respond. And when horses come and go frequently, you see the difference in horses that have been handled by professionals, who are expected to act like they have common sense, as opposed to those who only are handled by an owner who spoils them, or lets them get by with dangerous behaviors.
Big difference in how much a training barn will put up with, compared to someone who babies horsey.
Which is why I don't have much patience for people who can't get their horse to stand still for fly spray, or bridling, or what have you. Because if that horse was being handled properly, those would not be issues.
I also think that the more breeds and disciplines a person is exposed to, the better horsemen/women they are. That gives you better insight for dealing with horses right there.