So what should the Raiders do?
If the Raiders are indeed interested in bringing in Marcus Mariota as a true backup and a bit of competition for Carr then the positives far outweigh the negatives of this move. Mariota is a veteran QB with five years of experience and a pair of playoff appearances as a starter so he would be an immediate asset to the team.
As we have discussed, he is better than any backup QB on the Raiders in 2019 and would improve the position group as a whole. He is also the best option available among the free agent quarterbacks who could be looking for a backup role. In case of injury to Carr, Mariota could provide valuable cover and has proven that he is still good enough to win games in the NFL even if he has struggled at times.
The only reason this move would be a negative is if Gruden and the Raiders view him as a replacement for Carr because for reasons previously discussed, there is little reason to believe he would be an improvement. Some Raider fans have convinced themselves that Mariota could be the next Rich Gannon if he links up with Gruden and that would be the absolute best case scenario.
The more likely scenario is that he comes in, does his job in a professional manner, covers for Carr in case of injury or ideally in mop-up duty, and finds gainful employment elsewhere in the league in a year or two. If Marcus Mariota does indeed don the Silver and Black next year then he will be the best backup quarterback the Raiders have had in a long time, but he will not be the best quarterback on the roster.