It didn't do anything for me. Absolutely nothing combattext-like is showing up. :(
Maybe it's not doing anything for me either..I only noticed I was missing incoming damage messages because I don't have it display my outgoing damage messages. However, I do get the entering/exiting combat messages, and loot messages. I guess that's all I've noticed today.
These things all worked in at least uh.. version r74178.
Edit: reverted back to 74178 and all damage messages are working again.
In versions r74360 and r74331 I do not get any incoming damage messages. What I mean is the damage a mob is doing to me, or environmental damage(falling). I do not have any errors in bugsack, so I don't know what's going on.