And letting him slip to the 4th shows you that they were not sure about him. If they were, no way they would have let him get past round #3. They obviously liked Benn better.
No, you dont get it.
He would have been a terrible pick at #3, regardless of his rookie production, because he could easily be had in the third round or later.
The niners had Joe Montana as the #1 QB on their board, but they didnt take him in the first round, because that would be a terrible value pick because they knew they could get him in the third.
It's all about maximizing value on draft day.