We don't actually have any clue what Robinson is like at League One level, because he's literally played zero minutes in that competition. So if you're a Derby youth player, and you're looking at stuff like that, and the way Weston was used (or wasn't used, to be more precise), then what are the pitfalls of leaving? You may as well go and not play for a Prem club's first team than stay and not play for a League One team's one. And the worst case, you end up out on loan like Delap and make a decent Championship career out of it.
It's not even about whether Robinson is actually good enough *right now*, it's about giving him the chance to turn into a player that's good enough in the future. Max Bird's early appearances were pretty poor too, but we stuck with him, gave him chances, let him develop, and he's turned into an excellent player.