This is drifitng away from the original question, which was do we need registrations at all?
Imho yes, we do. You only have to look at some of the responses here and a dozen other threads about deps/ringers/cheating to see that a lot of people do still think some form of registration/ID & checking IS necessary. And as long as we need that, then I think the current system does the job just fine - registration cards are a photo ID for each player, any anomalies can be checked on or before the contest day so no bands are hanging around for days or weeks afterwards to see if their results are going to change, the cost to individual players is miminal, and only 1 person in each band has the extra stress of remembering the cards and looking after them.
Whether or not the rules attached to the registrations could do with some amending is another question...!