I've bought every game through DD on Vita, with the exception of MLB and that was just because you got a deal when buying the physical copies of the PS3 and Vita together. The reason I like to go DD (and why I will on the 3DS when that starts as well) is that I like being able to have a ton of games with me and not have to worry about digging out additional carts to swap them. If I get to a point where I finish a single player game and don't want to play it anymore or run out of storage, I can just back it up to my PC (for the Vita, anyway...not sure what my options will be for 3DS) and remove it to make room for more games.
I do a ton of trading in of games for my home consoles, but am willing to forgo that extra money just for convenience. If I was hurting for money, I guess I would think differently, but since I'm not, I can make this choice.