This is the only reason I would want a DSi. I'm not interested in the camera, I already have a portable browser, and the other bells & whistles just aren't unique to the DSi. A few more milimeters of screen, after all, is not worth the extra expense.
There is no reason even Nintendo would create an SD slot, announce a game download service, and then make you download (or transmit from Wii) every single time you wanted to play. And let's hope they don't double-dip... (e.g. if you wanted SMB3 for both Wii and DSi). They'd be justified, certainly, from a business perspective, but wouldn't that be a bonus if some Wii VC titles could be swapped to either machine from a single SD card?
That's why I'm waiting for the DSi. I figure in a year or so, the download service will be in full swing, and there will be plenty of reasons for me to give up GBA compatibility. GBA games will be available from the download service, sure, but why should I buy them again just to play them?