For messing about down the woods? Why not. It's only adding 250g of weight in a fairly inconsequential area. The type of bike it's fitted to doesn't really matter, if it improves the quality of your ride.
For short course XC racing? I think for technical courses they will start to feature more and more on XC race bikes, as they will probably save more time than the additional 250g of static weight will cost. For more boring courses or for those with the skills of Niño Schurter or Julien Absalon, they won't really matter.
For longer XC racing, such as endurance events (ie. Mountain Mayhem, Bontrager 24/12 etc.), I see them as something else to potentially go wrong and cause a problem during the course of the race. I removed my Reverb from my bike for mountain mayhem, and good thing too, as next time I put it on the bike it stopped working and is now winging its way back to Fishers for a warranty job! Less of an issue for a short course race, where if anything goes wrong you've lost it anyway, but in endurance where reliability is key, I don't think they're worth it.
My "XC race" bike (my everything bike now) will have one fitted for most of my riding though when it returns from warranty. Will still keep a rigid post too though.