Obviously, the much more logical way to do this would be to establish the qualifiers, then meet behind closed doors to establish seeding. This would allow for late player movement, injuries, current form, etc.
The reason UEFA plays by the numbers is simple but sad: Nobody wants to be accountable, or put in a position to be second-guessed.
I would ask this though. Based strictly on the "eye" test, who would favor the current Celtic team over the current Arsenal team in a 2-match tie?