I feel the odd need to defend digital copies. I do buy a physical copy of most games I buy, but digital copies to me fill a specific niche even outside of digital-only games. I usually only bring 1 physical game with me at all times, that one being the one inside my 3DS, because deciding what other games to bring seems pointlessly time-consuming (especially since I don't often play when I'm not at home, I mostly bring my 3DS for Streetpass stuff) and it would mean having to divide my rather sizeable game collection and always taking some sort of carrying case with me as well. Now, there are a few games which I want to have with me at all times, these include popular multi-player games (say, Mario Kart, which sadly wasn't available digitally when I bought it), long-term games which do not require constant attention (I don't play Animal Crossing, but it would be a perfect example, actual examples I do own are Fire Emblem and Pokémon, both of which I did buy digitally) and of course games which can be used to kill some time (which includes most digital-only games, Pushmo is a personal favourite, but the original Super Mario Bros has been filling that niche quite nicely as well).
As an example, I bought a physical copy of The Legend of Zelda: A Link Between Worlds (which was amazing and you should buy, like, yesterday) because when I play a Zelda game it's a dedicated project to me: it'll be the only game I play for any significant length of time until I finish it. However, I bought a digital copy of Pokémon because this allows me to train or breed a bit when I have the time would having to plan to bring the game beforehand, and it would theoretically allow me to trade and/or battle someone in real life at all times. I know a lot of people just take their entire collection (or at least relevant multi-player games) with them at all times, but I personally like to travel light and having to take out my the cartridge of the game I'm currently dedicated just to quickly do a Pokémon battle seems too much of an unnecessary hassle to me, even when at home and within arm's reach of my cartridges.
Chaos Jackal said:
Loading up the SD card only to delete something after some time is a total waste of money and space, especially if you have access to the physical copy.
You could of course just buy a reasonably sized SD card, having to delete a game seems like it would get rid of the only real advantage digital copies have.