No reason? Really?
Two simple yes or no questions for you:
1#. Do you think dirty oil and metallic particles are not remaining and clinging to engine parts after you quickly screw the drain plug back in?
Yes or no?
#2. Allowing engine oil to drain thoroughly does not help eliminate a good percentage of dirty oil and metallic particles?
Yes or no?
Yes or no?
Never had a problem with it. I didn't say quickly, I said wait until you no longer have a stream of oil, and that it drips. Usually about a drip every 15-20 seconds. That all depends on the oil temperature and recent use. That's also why I said to do it on an engine that has not been recently run.
The best way I've found is to drain it after it's sat, but still warm - an hour or so after driving, or even overnight in warm weather. There are spots in your engine that will never gravity drain completely - tops of rocker arms, little valleys, etc..., so it's overkill, you're still going to mix old with new.
A good filter and quality oil will do you better than wasting two hours of your time. Between my dad and me, we have over 80 years combined (50 for him and 30 for me) of anecdotal evidence that this works. Never lost an engine due to oil problems.
#2. Allowing engine oil to drain thoroughly does not help eliminate a good percentage of dirty oil and metallic particles?
Yes or no?
See #1.