Shoulda listened to JBot, he's been at it broken anything hobbit down to component atoms.
The reason pulsating (AKA half-wave, in this case) DC was used was simply that Honda in the 70's was still heavy on a kick of using DC for ignition. No idea why. Sucks, as the system was carried over from full motos which really suffered from that system.
That said, the coils really don't care too much, despite saying "DC" no them in some cases