He is an unkonwn, so no reason screwing up the next 5-7 years of the franchise for him.
He's an unknown to you, but the organization has an idea as what's going on with him. It is a risk, no doubt about that. Even the best scouts in the world have been wrong.
But you have to look at it this way: if you are the GM/Coach/Scout/whatever in charge of this decision, if you think he is a stud QB and future MVP, you have to take that risk if it is available. Why wouldn't you? Hands down, no questions asked, QB is the most important position in sports. You can win without one, but that is the exception to the rule. If you want to put yourself in a position to win a championship or two...you start with a franchise QB. They don't have one now (and didn't in Foles). They can wait and maybe grab one somewhere else...but if you think Mariota is that guy, you have to take the chance. If he flops...that sucks and the franchise is set back for sure. But if you wait and hope that a franchise QB falls to you at some point...you probably aren't winning anything anyway.
I don't think they should trade for him just to make the trade. If they aren't sold that Mariota is the real deal, slam-dunk, franchise QB, then don't draft him. But if they think he is...how can you not make that move? The consequences are bad if he fails...but the reward is huge if it is successful. And again, are they winning a SB with Bradford? Foles? Hundley? Billy Volek? Probably not. So the team is still 4-5 years away from winning anything any way.