I personally don't think that having a separate friend code is because of the D/P compatibility. Nintendo never actually officially announced that only 1 code will be required. Sure, yeah, Elebits uses the Wii's Address Book for WC24, from what I've seen, but it's only a possibility, it's not guaranteed.
For now, the code is only for sending messages to other people on the Message Board, or for trading/putting Miis on Mingle, or if you have it, Elebits.
The reason why Nintendo didn't release online games on the fly at launch day is unsure, maybe because it wasn't developed in the early stages of the Wii, or maybe because Nintendo's having a hard thought of what to add to the service next. There's a possibility that we may see fees from now on, but I still think that's got a very low probability as well because Nintendo clearly stated that the WFC is free. For now, Nintendo is all over the place, and we'll see if in 2007 it'll all go back together. I hear Nintendo is having trouble especially with Super Smash Bros. Brawl mainly due to the online feature, but hopefully that gets back into place. It's certain that 2007 will be a great year, but not so sure when the real Revolution is going to start for hardcore gamers.