The feat definitely is limited but not wholly worthless. Consider this: A cleric could take this feat or take the feat that gives a penalty to turning damage but grants a greater chance of turning a higher level undead (sorry don't have books with me right now - forget the name). It seems to me that any cleric worth his salt would be able to turn most undead anyway and would never even get to use this feat. However, if facing an undead with turning resistance, lots of hit dice, or the cleric just has a low charisma (or any other reason why the turning check sux), then this feat might be useful as damage is an easy substitute for defeating the undead.
On the other hand, a paladin, with a crappy turning ability (considering he is always two levels behind) can put this feat to much better use as the added damage is much more useful. Unsure if they errata'd the PrC in defender's of the faith - the one that is the undead slayer (sorry - forget the name again), but this feat would also prove tremendously useful to him or her.
WOuld I ever take the feat - probably not. Is the feat, absolutely horrible - not quite.