Simple.
The DS RAM you are downloading the game is only so large. So you could pretty much only download one multiplayer only game at a time. You won't be getting the entire game, and you will be limited, because once you turn off the game it is over.
Besides that I am sure that you are going to need at least one system with the full game to be constantly sending new information over the WIFI, like the map of the next level you are going to playing at and stuff.
So really this isn't going to be a problem or likely.
Could you someone figure out to do this, yeah, but if it is a section of the game Nintendo is allowing to be copied via wireless connection, then I am willing to bet they would like you to do that and play their game (and potentially buy the full version) then not play the game at all.
This is not a problem. Nintendo has probably looked at this threat entirely.
However, it does atleast allow the homebrew people to design games for the DS, and potentially find away to actually get their code to the DS, to share with gamers around the world, and that is cool.