It's more about portability. I think we ALL love to play on as big of a screen as we can, but reality is that unless you're a hermit, there are lots of times you don't have that ability: like long trips, plane flights, waiting rooms, visiting the in laws, breaks at school/work, bar mitzvahs, weddings, restaurants, ********prison********, funerals, waiting in line at the DMV, on the crapper, the list goes on and on.
I could see a MMO tied to your normal game on a phone becoming a problem for alot of people. Most MMO's require quite a bit of devoted time. For one, you're paying a fee to use it, so (at least for me) you feel inclined to get as much use out of it as you can. Plus, since it is a ever changing environment that requires some sort of daily if not hourly interaction to keep up, you feel the need to be there. Having it at your fingertips everywhere, well...you see where I'm going. Not everyone is like this, but I would gamble that people playing the same MMO solid for 3+ years would have this problem.