3GP is todays standard for mobile devices (or, to be more specific, 3G mobile phones, but for the reason, 3GP support is the most important standard in all mobile devices).
MP3 isn't a set standard for devices, like 3GP/MPEG-4/AAC.
Not having MP3 playback in DSi (or Wii for that matter), is an issue of cost, but considering the format is going to be obsolete and all devices should support AAC (it's also the format most consumer devices record the audio), i don't see much point in having MP3 support (outside i being the most commonly used compression today), if we are talking about tradeoff between MP3 and AAC.
As for Wii supporting AAC and not MP3, the idea is, that you transfer your files from your mobile device to Wii, that record to AAC format, not to play your pirated records. The compatibility is much more better.
Ei Kiinasti.
Eikä Japanisti.
Vaan pannaan jalalla koreasti.
Nintendo games sell only on Nintendo system.