I've been following Rohan at Blessing of Kings for some time and usually find his insight into MMO games to be well thought out and often accurate. His latest post on the inherent problems facing MMO games in relation to its player base (transient and extended players), and thier raiding habits is a good estimate into the problems when you try to merge two different types of players into one format.
The single biggest problem with the endgame of WoW is that it persists in
believing that if the incentives are just right, Transient players will
transform into Extended players, and everything will work out properly. But this
transformation never occurs.
I encourage everyone to read the whole post to fully understand his point of view and the subsequent debate occuring in the comments offers some good points on the subject.