I'm doing prototypes because I try to create a fun game. If I don't think that it's fun, I stop the development ("Make a great game or kill it early.").
I will finish my sidescroller definitely, Superku: Skate probably too.
I've created a (4 player) Tennis game before the shooter prototype, but it wasn't fun, too, so I've stopped it.
That game wasn't so laggy as the soccer game, though.