You don't take Brandon Weeden at 8 for the same reason you don't take Ryan Mallett at 15. The NFL has settled on the value of Weeden and it's significantly below #8 overall. To me when it comes to this position you decide who you want and then you pay the market price or something a little above it if you want to take risk out of the equation. Taking Weeden at #8 when he's set to fall all the way to #41 overall, for instance...that's not "a little above" market price.
Now, if the market were telling me they value Brandon Weeden in the top 15, that's a different story. He'd be the exact same player, I'd have the exact same evaluation on him, and I'd take him at #8 overall.
Draft values are never absolute, IMO. They're always relative.