"Because the safer and more reliable alternatives weren't appealing, at the time. The removal was only temporary, and it's something a person can do for his or herself," Sirius says, since Q had already ruled out Legillimency and hadn't seemed particularly enthusiastic about the imaging. "And while it isn't necessary now -- at least, not if we can make arrangements for the imaging -- I was curious as to whether it had actually revealed anything."
I think the possession stuff is mostly until they actually have the equipment -- could steer them back onto the Animagus discussion?