You should be doing it once a year if you drive a lot. Fluid changes are hella-cheaper than component replacement. AMC has a good point... coming from a member of the automotive industry: you can't ignore that facet of ownership. Everything will break down eventually, so you gotta read the maintenance schedule and actually follow it!
It happens regularly enough: car comes in with shifting-woes, gets a loooong overdue flush and DOES start acting different. It was too late (like differential whine; if you're starting to hear it, it's already funked-up) beforehand, but the customers act/say the dealer/shop/mfg is out to get their money!
The car cost thousands upon thousands to buy; what makes people think any one part of it is going to be cheap?!?! The techs and mechanics don't put those miles on it...
Pick up a case of fluid with your tax money, swap it out every year or so in the driveway... save your trans!
Coal Country Cruise... YOU MISSED IT!!! Pics here: http://www.focusfanatics.com/forum/showthread.php?t=296590&page=2