Well with root access you can have a ROM that is faster and more stable that the stock one, allowing other people to fix bugs that google and HTC don't fix.
But of course its not all about the roms. You can back up your entire phone, like your pc and restore it exactly as it was, even if it was wiped. You can control CPU speeds to give further performance increases without wasting additional battery power or run it slower to save lots of battery.
You can control applications at start up and prevent them from doing some of the things they want to do. You can even update the radio to newer versions to get better network signal.
You can uninstall apps you don't want that are pre-installed on the ROM
Once you have this (and everything else at your fingertips) and allow the time to explore the posibilities, unrooting just doesn't make sense. You could have always got a rooted stock rom, then had the option to utilise it when you knew what it was you actually had.