Rooted with a stock ROM and custom kernel is a great setup for those who aren't really looking to customize their UI but want to have access to all that the hardware offers. HDMI mirroring is a fairly recent feature. You also get:
much improved touch tracking sensitivity
5 pt multitouch
ability to wifi-tether or USB tether for free
ability to over- or under-clock your CPU
take screenshots
put apps completely on your SD card (a2sd)
make NANDroid backups/restores (complete image of your internal memory)
use your own splash screen and boot animation
remove the bundled carrier crapware
send/receive files wirelessly to your phone
set up your phone with various servers: web server, FTP, VNC, ssh, etc.
and lots of other stuff I can't think of right now
Of course you don't need to utilize all of this stuff. but it's there if and when you're ready to try it out.