Doubling-Up was originally set up to allow young brits to gain experience in the higher division/league, while still riding regularly in the lower division/league. That is great as a principle, but it was allowed to get out of control, so that any rider (depending on converted average when doubling-down) could race regularly in both divisions/leagues - so much so that there have even been accusations of senior riders dropping their top division/league average so they they qualified to double-up.
If you can't see a problem with any of that, then fair enough - as always, we makes our choices.