User blog comment:Aegis Runestone/Revelations and Brotherhood--Delay for AC3?/@comment-4705174-20120422175029/@comment-4975789-20120423193544

AC:B and AC:R were needed, at least, to help continue Desmond's story. As Assassin75ibn said, the team were escaping from Abstergo, so couldn't go directly into either saving the world or taking down the Templars. Ubi couldn't have done more on Ezio's story in ACII as either it would feel rushed, or it would make the game so much longer. Therefore AC:B was needed, to both finish Ezio's story and continue Desmonds.

Ezio's story in AC:R wasn't needed though, nor was Desmond's coma after AC:B. Ubi could have just created a new plot twist to continue the franchise. So AC:R was either a ploy for Ubi to make more money, or, as the blog says, a way to stretch out development time on ACIII.