I have a smart tv, only cause most decent tvs nowadays are tricked out with BS stuff I don't want. HTPC>>>>>>smart tv.
Some folks like apple cause everything just works. Some folks like to tinker and get things working the way they want and generally go with pc.
If you want something to just work and not have to spend any time on it then a smart tv is a good option for those who just want to watch a show on netflix or on a external drive. A lot of people with HTPCs want the nice front end you get with media browser or XBMC. They also like to customize all that stuff. I can't have a nice XBMC aeon neo interface with custom categories, custom artwork, nes, snes, N64, ps, ps2, dream cast, game cube emulators, over 600 games, over 300 movies, over 2000 tv shows, acess to steam pc games, and any webpage on the internet with a smart tv.