Steve,
You are wrong the sliding portion of the caliper moves as the pads wear, this then stresses the brake pipes. So when you put your foot on the brake the pipe is pulled outward, and when you take your foot off the pipe will act like a spring and unsettle/pull the sliding portion away from the disc. bear in mind we are only talking half a mm at most but its enough to give you a nasty peddle. The other potential long term consequence of this is the eventual work hardening of the brake pipes and possible failure.