I think McD and Beane kept him around to be on the PS but his TC play combined with the horrid play by AJ forced their hand. Honestly, I think it says more about what they thought of him that they were willing to start guys off the street with a week of practice over him twice in a four week stretch.
Gruden, on the other hand, picked up both EJ and AJ after the book was pretty much written, soooooo... maybe he just has a thing for really bad QBs.