Why my primary system is my PC:
-I like being able to see my hardware "at work" and I enjoy tweaking, maintaining and learning more about my computer.
-Every game looks better on it. And I can still play games with a controller on my HDTV if I want to (which I do--haven't been able to switch back to my 22" monitor since I tried it, funny that)
-I can easily get an unconventional entertainment solution (TM) going quickly. Such as attempting Street Fighter 4 combo trials while watching Netflix, with a VesperArcade SF YouTube tutorial open and ready to check in case I can't figure a combo out. All comfortably on the same screen.
-Faster loading times (sometimes drastically faster) means I've spent an extra year actually playing games that I'd have otherwise spent watching a spinning icon
-I don't need 60+ FPS all teh timez!
but above 30 is really nice. In general, stuff looks and feels better at higher framerates.
-Better multi player and things like Ventrilo and Skype. I don't have to pay for P2P crap matchmaking, I get that sludge for free
-There are other raisins in my Reason Bran, but I'm stopping because my posts always get really long.
I also play on a PS3. Mostly my free copies of Little Big Planet 2 and King of Fighters XIII (PS+ is really good for someone who doesn't have many games). And of course The Walking Dead episodes when they come out (when I bought, the $20 for all episodes on PSN was a better deal than Steam).