Every single player across every single server who swaps gear has a decent chance of being a better DD so it depends how you define bad.

I've been a main tank in many incarnations throughout my time playing the game and while being the focus of healer attention have gear swapped extensively. I have never to my recollection died as a result. I've lost a handful of hastes and a handful of cures but that's about it - I've never died from it (again to my recollection) and it improved my efficiency tremendously.