well what is it?

And is it an advantage? or just something unique.

Also what's the advantage of a single swingarm?

The card ignition has i think a chip inside and is part of the ignition when inserted there are i think 3 types of card standard(restricted) dry race and wet race these race cards disable all road going features lights etc.It was Hondas way of stopping hairy Jap kids from derestricting their bikes but there is a splice for this on the TYGA web site.No performance improvement over the MC21 but it is kind of trick!.Single sided swing arm was fashionable in the 90's endurance racer look but heavier than conventional set up.Thats about it.

Can you find a credit card ignition assembly and have it installed into your own NSR?

Is there any point to having a credit card ignition on a street bike?

Street Cred. No other reason. Honda have not used the technology on any other bikes (as far as I know) and no-one else uses it today. Nice try, but not really worth it in the long run...
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

Do you still need a key? or is the credit card the only way of turning on the bike?

MC28's, the bikes that use the electromagnetic data card(aka creditcard) do not use an ignition key. Just the card. They do use a key for the fuel tank though and it's a small key that fits into the corner of the data card.