It seems to me that the OP is really talking about having one card with your highest certification level plus your specialties. For example a Rescue Diver with EANx, Dry Suit, Altitude, Doplhin Atlantis and so on.
I can understand the idea. The issues are going to be:
1. Higher administrative costs, making the card more expensive.
2. Sufficient demand on the part of recreational divers to make it a worthwhile enterprise.
3. Whether this should be planned to eventually replace C cards with separate specialty cards.
I am not sure this would catch on as an idea. The present system seems to work OK. Most people walk around with a driving licence, social security card, a stack load of credit cards, membership cards, fidelity cards and so on.
Another post talked about being able to choose which card to show at any given place. That is one of the beauties of the system, especially for dive pros who want to go on holiday.
This reminds me of the Blackberry. It's a great concept but still a whole load of us rather have a normal cell phone and a laptop. Lot of Blackberries out there, however............