Your argument is sound. However, I personally don't play for more than a couple of hours at a time. I don't have time to sit around trying to get a PuG going. So, five boxing heroics or doing dailies to buy gear works out for me. If I did play like I used to (say 6-8 hour sessions) I wouldnt' even have to do heroics for very long [heck, even on my current schedule I don't really need to do that many]. There is no reason to, well besides for fun and the sense of accomplishment. There is a ton of pre-raid gear you can buy or get from rep [at least for the classes I play] that prepares you for Naxx. I also get to play the spec I want.

It also turns out that Naxx isn't all that difficult [never been, but that's my general impression] so things have changed since BC (pre-Kara changes etc) where PuGs now have more success (at least that's what it seems like). Also, my experience in BC was that people were very much unwilling to carry anyone through an instance or raid. If you didn't pass a gear check you weren't going. Maybe that's changed, I dunno.

Also, keep in mind it is now late in the game. People are pretty much geared out, so hitting Nax etc as a new toon will get you a ton of gear.