The other point is let's say the shit hits the fan, Stafford gets hurt in week 2, Bennett is overmatched, Simpson shows flashes, but isn't ready and the Rams win 5 games, end up picking 6th in next years draft.
There would be nothing preventing them from taking a higher rated QB and holding onto Simpson for a yr or so and eventually dealing him. He would have value, assuming he plays well when he does play.
So I don't see a downside to the pick outside of yes, you could have taken a guy who could be more of a factor this year in Lemon. Had they taken an OT, that is another position that would have been more like a backup role for this year.