If you talk to your LFS, you will know about people rushing their fish to the store because their tanks separate from the seam and turn their place into a wading pool. This is not too common but it does happen from time to time.
Just because it hasn't happen to you, doesn't mean you are immune to it. I personally had 'used' tanks that leak. I even had a couple of tanks that cracked (side and bottom) due to uneven support. When you are keeping 50+ tanks, you are much more prone to these types of disaster. One busted tank is one too many.
Personally, I have stopped using glass aquarium over 10gallon size and switched to arcylic tanks and they are alot stronger and more forgiving. I still have about 20 or so 10 gallon glass aquariums that are 20 years old and a few of them had leaked before.