I'm not a high rank crafter yet, but I repair everything I see and am able to repair, because I remember how I hated waiting for someone who repairs my stuff before I found a nice LS who helps me out on that.
But obviously I only repair it if the reward is the same amount of Gil (or more) the dark matter costs at the NPC, because I don't want to pay ADDITIONALLY for someones stuff to be repaired. It would be different if crafters got a lot of SP for repairing stuff, but we don't, so repairing is not really useful in any way but making others happy.