I thinks any good MMO has elements of both. In fact, I believe that both casual and hardcore players need each other in an MMO so the best MMO will have elements for both groups.
The hardcore gamer invests a lot of time in the game and are challenged and rewarded. They have the best gear and coolest experiences. They inspire the casual gamer to play more.
The casual gamer will play in short bursts, but they spend money on items and help the market move. They generally don't spend time in crafting so they need the hardcore players for gear.
This in turn helps the hardcore out.
That all being said, content should be created for both groups, that way player x who logs in and only has 30 minutes to an hour to play is able to enjoy themselves, but player y who can play 4 to 6 hours a day needs contents so they don't run of of things to do.
It's a hard balance, and I hope that when this game is finished we will have that balance.