Some games ONLY have download play as an option.
These are the ones that the flashcart makers should focus on getting compatibility with first, then the others.
Well, I know Mario64 and Bomberman who have that, and both have download play working with most flashcarts.
And I use Download Play yeah, my friends don't have many games for the DS, so I use the download play to multiplayer games like Mario Kart with them, I have the real game myself though, my current flashcart doesn't support mk, but I think it's quite handy to have download play supported.