I think if the argument is that wizards can all reach the one place easily via Floo or Apparition or anything else, then they can reach other hospitals just as easily as well. If it's a matter of centralising so that all the specialists can be found in just one place, you can still say the physical location of everyone doesn't matter because there are easy ways to travel in between, right?

Can you imagine if St Mungo's were the only hospital around and then it got attacked? Hogwarts' Hospital wing would be inundated!

Evyclaw