If a page has more <ref>s than </ref>s or vice versa, that's a clue that
something's wrong. If something *is* wrong, you're probably going to get either
a huge chunk of text stuck in a reference or a reference stuck in the middle of
a page, which are both bad (see the URL for a typical case of the former).
Furthermore, people who aren't technically-minded will probably have not the
slightest clue how to fix the problem--I've seen this happen twice. Since
there's no legitimate reason anyone should ever want to save unbalanced <ref>s,
I suggest an error message be presented when they try rather than allowing them
to screw up the page.