The argument of cracking the bleeder is to avoid pushing dirty fluid from the caliper back into the system. Its does seem to be the dirtiest in my experience. Ok, that's is part of it. The brake fluid in the caliper reservoir (must be a name for it) gets super heated and turns black. You don't want that stuff getting pushed back into the lines.
The main point of cracking the bleeder is to reduce the line pressure to ZERO. Consequently, when the bleeder is cracked one can push the caliper pistons back in with one's thumbs. No need for high tension special tools or clamps. It takes wicked pressure to force all that brake fluid backwards into the main reservoir.
If you can't crack a bleeder or too nervous to do so, you should be doing a brake job.