I'd modify that to "not talented enough to play... in a drop-back passing offense." The problem isn't that you couldn't win a lot of games with him as a backup... it's that to use his talents optimally, you'd have to change your offensive system dramatically when you put him in.
For the life of me I will never understand why the Browns drafted Johnny Manziel — or why Manziel is still on the Browns roster.
For that same reason.