Why would you want a flashcart over a softmod?
A flash cart would be the only way to play commercial games since the SD card slot is not as fast as the cartridge slot.
What about emulators for lower-tech systems (like NES, SNES, Genesis/MegaDrive, etc.)? Will that be possible with an SD card?
On a side note, I know that many people will tear me apart for saying this, since there are lots of people on GBATemp that support piracy, but I personally hope that it will remain impossible to pirate 3DS games. I really hope the 3DS won't become a sickening pirate-fest like what happened with the DS.
(oh and I don't consider emulations of really old systems as piracy)