I'll throw my penny's worth of thoughts into the ring and then leave quietly.
After the original GB (I still have mine) came the GBC and the *new* improved games that took advantage of the colour features, but the GBC was still compatible with GB games.
Then, after a couple of redesigns, came the GBA with *new* improved games that took advantage of the new features, but the GBA was still compatible with GB and GBC games.
Then, after a couple of redesigns, came the DS with *new* improved games that took advantage of the new features, but the DS was still compatible with GBA games but not GB and GBC games.
Then, after a redesign, comes the DSi with *new* improved games that take advantage of the new features, but the DSi is still compatible with DS games but not GB, GBC, or GBA games.
It's a process that has been happening since the GB was first remodelled. Once Nintendo has sold enough units there may well be *must have* DSi only games to encourage people to ditch their DS and DSLites in favour of the new handheld. There is also the possibility that the DSi Shop Channel will have some decent releases (rather than stripped out versions of previous retail releases).
I am surprised, as are others, that there is not a single DSi only retail title released yet.