1) Convenience
Backing an HTPC w/ a server and ripping only the movie and preferred audio means watching anything with just a couple button presses. No warnings, menus, coming attractions, etc.
Using my tablet (netremote, girder, usb-uirt) with nice big easy to see, easy to press, completely intuitive and fully automated or use my wireless headset to execute voice commands is the best and easiest remote control gets. I have tailored my remote control/automation to exactly reflect my desires.
You could watch 10 movies in my home and never even know you are using an HTPC.
No shelves full of DVDs taking up space and risking damage. The entire DVD experience is completely invisible.
2) Repairability
No matter what breaks, I can troubleshoot the problem and repair it myself. I could not do that with a STB and cannot do it with my audio setup or dish setup or TV. I wish everything worked like my HTPC. Sending anything for repair is a MAJOR PAIN and very expensive.
3) Upgradability
For either no money or relatively little money my HTPC keeps getting better (improved PQ, functionality, convenience, etc.). No thowing out and buying new.
4) Fun
Reading the forums, getting new ideas and trying them out is fun. I like it.
5) Uniqueness
I like being the only one on my block to have such a setup. I also like being the only one on my block to have the knowledge and capability.
6) Functionality
All the things that a single box can do and the price at which it can do it ... DVDs, PVR, Photos, home automation, music, etc, etc, etc. Plus I can do PC things on the big screen - Email, net, word, games, etc, etc, etc.
Sorry, these are the big 6. I believe I've calculated 22 reasons in total.