Not arguing for or against it, but the logic is to prevent junior pilots from entering a category that they couldn't normally hold by coming in through the backdoor while more senior pilots are left out. Take a look at the IAH 777 CA roster, for example. The most junior person currently in that seat who got there via a vacancy bid is around #2500, but there are 8 pilots junior to him, all of whom got in via a base trade, whose seniority numbers go all that way up past #4000. If you're the #2600 guy and you've been waiting 10+ years to try to get into that seat, you might be upset that people junior to you are getting there first and filling those potential vacancies by "artificially" getting on the equipment first and then sliding over. The new policy still allows for base trading, but it prevents the #5000 guy from bidding SFO 777 CA and then base trading back into IAH while the #3000 guy is patiently sitting on the sidelines.

Of course, under the old policy, the more senior pilots could pull off that same maneuver. But there's risk there, and many of them don't want to be commuters for an uncertain period of time, so this was put in place to make sure that those entering a category have the seniority to hold it "naturally" rather than working an end-around.