I now know why the "Add Template Name In HTML Comments" are a serious (!) security vulnerability.
With an unpatched board with this feature enabled, a cracker can inject malicious PHP code (yes, ANY PHP code) by the use of a malformed URL.
Of course, I'm not about to state HOW this is done, but let me just say that if *I* could find it (and I wasn't even LOOKING for this info!), then a cracker with a grudge will surely find it.
I hope this helps to make users patch themselves, if some are still in doubt of the severity of this exploit