That's not entirely true. Whitehead, Warford, Martin, Riddick, Levy, and Hill have all been great values taken after the 2nd round. Hill got out of dodge, but the rest are (or will be in Riddick's case) significant contributors. Mayhew can find players later in the draft, but it's a toss-up.
Trading down to get more picks makes sense only if there's no player, at the time you are picking, that you really want. Trading down when you are giving up a player of need/value is just dumb.
Example1: Let's say Detroit targets Cedric Ogbuehi (OT) as the best player on their board when they pick. But another team wants to trade up from the top of the 2nd for someone like Gurley. Detroit should know Ogbuehi is likely to still be around at the top of the 2nd, so getting an extra pick with the player they want is a smart move.
Example2: Same basic situation, but let's say Andrus Peat (OT) falls due to concerns that he's only fit to play RT. Peat is considered a significantly better player than Ogbuehi, and we would be using either one at RT in year one anyway. So when another team calls to trade, even if they are looking for Gurley, it would be dumb to trade down and miss out on Peat.
