The reason that GBA slot is in the DS (or one of the reasons anyway) is because some games will give you unlockables based on the cartridge inserted. I've also heard a rumour of some games coming out with their own extension type things that fit into that slot.
I'll admit, it would've been nice if the GB/CGB feature was intact but I could see why it may not be. I don't think that those are the majority of the games played anymore, or that they are in stores (well...they might...maybe used or something). So, why put a relativley useless feature. Myself...I haven't really played any of the older games recently. Exception would be Super MArio Bros. Deluxe because it is better than the NES Classic version. But I can live playing on my AGB and Game Boy Player. I see no great benefit from playing it on the DS either.
I think something may be possible with ROMs and flash cartridges. I think I read something, but I couldn't figure out if it was a rumour or not.
No...I don't think I'd buy one. I only play Mario and Tetris. I won't buy an extension just for two games (I play my Zelda ones on the big screen with the Game Boy Player)