Mentoring systems –such as those seen in games like EQ2, CoH and STO– are still rarity in the genre, despite being an obvious enabler to allowing friends to play together. Yet most MMOs implement some form of PvP where players of varying levels are slammed together, with the level disparity ‘normalised’ by temporarily increasing the level of the lowest players.
Thus the system is often applied to PvP, where lower level players are more likely to be put off playing again because they are dramatically behind the power curve of higher level players (due the significant differential in equipment, number of powers, etc), and are in direct competition with those players.
Yet the system is often not applied to PvE, where lower level players are more likely to be prevented from playing with friends if they are dramatically behind the power curve of friends who are at a higher level, and are thus unable to join them in a cooperative effort.
It seems to me, at first glance, like a curiously backwards convention of design.