After dueling someone or after being in an intense battle with a group of people, or even during one of those, I and others wonder how many hearts does(do) the other player(s) have? We ask ourselves this to know how good or pathetic we are. Pretty self explanatory.
My suggestion is to have tab show how many hearts players have. Countless other servers have this during their mini games and PVP, just because it's a nice feature.
So, in Summary: Add hearts to players in tab.