Diagnose these days is not as WORST as everyone think of. I'm a fan of this ring, I admit, and Diagnose has been my casual heal as for me being a sweeper and buffer.
The bright side of the ring glowed ever since DMS came- it became a must-have for Tankers. It can SAVE your crew from Night Frights (Huddle, Toggle then rage Diag). I saw this strategy done by three tankers I've met and it really worked.
Diagnose is also an alternative for Wish, if you were not able to target friends during a mob. It turns out that it acts as the 'first-aid kit' of the healing rings... IIf used properly, this First-aid Kit can save you and not sell you down the river.
Each heals has it's own characteristics: Bandage being the strongest healer, Wish being the most selfless, and Diagnose as the mediocre, yet all-around healing ring, which makes it unique among others.
Plus, who would hate that cool animation and catchy SFx? Another feature I love with Diagnose 4laugh
Additional pun: "I know Diagnose is a bad thing to have, but it doesn't mean it's a bad ring to have either..."