It is a treadmill to keep you subbed. However, the other way I see it... it promotes healthier game play. By limiting the player to 450 per week, it allows for the players that just want to run expert five times per week to be on the same level playing field as the player that wants to run 450 worth of weekly tomes on Tuesdays.

Healthier to provide options. Also healthier to encourage players to try other content. 24-man for a tomestone-equivalent piece (albeit, not upgradeable) and savage content, if that's their prerogative.