Brake calipers have seals, so it's a bit misleading to say that this fluid is dirty. further all brake fluid is not hydroscopic. (look at dot 5's chemical makeup). And I know several ASE master tech's that hang new rotors, pads and clean up the caliper pins. It's rare that they open the bleeders when just replacing pads and rotors. Severe side effects, that's a bit overblown. There is nothing wrong with doing what you are saying, other than it may be an unnecessary additional cost.