This has been brought up dozens upon dozens of times. It's not a good idea, because:
1. it doesn't reward teamplay at all, only individual achievement.
2. it doesn't reward people for supporting (i.e pike poking around teammates to let them get the hits in, or shielder protecting team from ranged)
3. we already have enough bloodlust that teamwounds and teamkills happen due to people rushing in to get an easy kill even with NO reward for it.
4. The rich get richer. Good players who have played a long time will be able to retire even faster and the gap between new and old players will grow even more rapidly.