No,just no.
First, he can't run the Pats system. For him to have any chance to be successful he needs to go to a team that designs their offense completely around him, that won't happen in New England. Because he can't run the Pats system he will be largely wasted as a backup because he won't be able to help our offensive skill position players develop in practice and during training camp. We had the same problem with Tebow, you want the guys who arent getting reps with Brady to get in similar type work with the backup but you can't really do that with Manziel. He will be largely useless as a scout team QB because he is so different from anyone else in the NFL, he won't be able to replicate a traditional pocket passer or even th larger running QB's like Newton and CK.
This doesn't even take into account the value of drafting a backup QB, the fact that I don't think he is a good enough pocket passer, his attitude and character concerns, the distraction it would bring and I just don't like the guy. I'll pass thanks.