Several reasons not to go with him.
First off, the Tiger-Cats are still reeling from a terrible attempting to sign Art Briles, and then double downing of the attempt. A man embroiled in the sexual assault scandal at Baylor. It was a repulsive signing attempt that was thankfully stopped, although seemingly because the league office was about to step in. So then to not just hire, but go out of your way to seek out a QB with previous drinking and domestic violence issues. The optics don't look good if you are trying to brand yourself as the family friendly entertainment option and use him as the face of your franchise and furthering that controversy.
Second, is the issue of he needs to be good, but not great for the team to benefit in the long term, which isn't something that's easy to build a Grey Cup team on. If I may, if Manziel stinks it up, well then you've disrespected your current starter, you still don't have a plan or options to get a star QB and you've lost a season. If Manziel does great, then he will be off to the NFL in a heartbeat, there is little question that he will go to the NFL and play second string to make more money if he can produce evidence that if off field issues are resolved and he's still got it. So at best, you'll see two years out of him. You've produced brief success at the cost of sabotaging your development of Masoli and any other prospect for those two years. You also don't have any gauruntees that he will win you a Grey Cup against league stalwarts like Mike Riley, Bo Levi Mitchell, Ricky Ray and various younger QBs who are proving themselves more and more and will likely be retained by their teams. Yes, a large number of players in the CFL want to get back to the NFL, but those that don't are the ones who have been around for 5+ years and have developed roots. Manziel seems unlikely, given how much he fought for an absurd salary.
Third, is you may or may not have overpaid for him. Manziel's contract is somewhere between 150-250 from my understanding, which is low for a starting QB but fairly high for a backup from my understanding. That in turn causes cap issues, which the Cats should be over will shedding the salaries of CJ Gable, Jon Chick and Zach Collaros. However, given the did literally nothing in free agency except sign a kicker, that may not be the case. His initial requests were completely absurd.
Fourth, is has he resolved his off field issues? If he hasn't, you've wasted a lot of time pursuing him as the CFL put strict requirements on him coming up here. Additionally, there is a huge and real possibility that you are inviting locker room poison into the room. Big names slumming in the CFL can cause friction. Look at Duron Carter in Montreal for example. If the staff start giving him preferential treatment (which they may because of his name, it's a sad fact but it has happened many times in the CFL) it can quickly kill your locker room and culture.
Fifth, while it is hard to find that diamond in the rough, the QB position has a glut of talent waiting for any opening. It's not brain surgery, there are 240+ NCAA first division teams and 27 CIS teams with 32 NFL and 9 CFL teams for prospective QBs to make a career. A ratio of 8.5 teams producing talent for every 1 pro franchise. It might be hard to find the right QB, but it's not hard to find a prospective one.