Let me first off say, combat rez is worthwhile, if you die on pure accident. I find that if a group lets their healer die, your in a **** load of trouble and a combat rez will still result in death a second time.
BUT - If you are smart, and you see an eminent wipe, and you get yourself positioned in a location where you can die and stay dead, then rez the group, it will most certainly help the group.
SOOO yes the points in improved reincarnation are worthwhile, and every shaman should have them. :)