In my experience, ST only automatically reloads a changed file if there are no unsaved changes in the buffer. If the buffer contains unsaved changes when the file is modified outside of ST, you get a dialog asking whether or not you want to reload the file from disk. (Note: my experience is on Windows - I suppose that other platforms may have different behavior).
So there doesn't seem to be much danger of lost work. But it might be nice to have an option that will notify you of a changed file before reloading it regardless of whether or not the ST buffer is 'dirty'.
Also, even though using version control is a good practice, files can change for reasons other than two users making modifications simultaneously. For example, I might be looking at the output of a program that I'm debugging or I might have edited the file in some other tool for whatever reason. There are legitimate reasons to want to be notified of changed files that have nothing to do with version control.