I was told by a GM transmission engineer (retired now) that if the trans is flushed regularly it is perfectly ok and recommended. Providing:
You use the same type of fluid that will be going into the trans (with GM Dexron) and never use a chemical flush (especially the stuff sold by the quick lube places);
And make sure it is a good machine; his recommendation at the time was a Kent Moore used in GM dealerships and again, stay away from places like Grease Monkey.
One thing a flush will do that a trans fluid change cannot do, is change out all the fluid. Depending on trans and torque converter, over half of the fluid is not changed in the torque converter, the cooler lines and the cooler(s). In the old days, the torque converters had a drain plug, new ones don't.
I had my Hummer trans flushed twice while living in Moab and never had a problem. However, like many, I have the trans fluid changed out quite often if towing or like with the Hummer; climbing rock steps in 115 degree heat.
But like the 3K oil change many will disagree, some will swear by a flush, some will swear at a flush.
I'll get flushes, if I ever keep a truck over 16K miles/two years.