I can see your point Michael. I only gave a Reader's Digest version, my fault. Trying to be extra quick while utilizing a bit of quiet time at work.
If cracking the bleeder while compressing a caliper piston, you have to be sure that only one piston is moving at a time. This is accomplished by using a plate mounted to hold the oppsing piston firm, or by leaving the old pad on the opposing side while compressing. Once the piston is compressed slip in the new pad and compress the opposite piston. This keeps the pistons from lodging in their bores. Can't say that it prevents contaminants from entering through the bleeder though. Cracking the bleeder prevents an overflow condition at the brake resevoir, something that can be avoided by simply siphoning off some of the old fluid beforehand, eliminating the need to crack the screw. I would also crack the bleeder screw only if flushing the system immediately afterward.
So many ways to skin a cat.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.