I think you've answered your own question. The biggest drawback is how they interfere with the memory leak detection tools, but I think that drawback is a HUGE drawback for certain types of applications.
I work with legacy server applications that are supposed to be rock solid but they have leaks and the globals DO get in the way of the memory detection tools. It's a big deal.
In the book "Collapse" by Jared Diamond, the author wonders about what the guy was thinking who cut down the last tree on Easter Island, the tree he would have needed in order to build a canoe to get off the island. I wonder about the day many years ago when that first global was added to our codebase. THAT was the day it should have been caught.
