The fact is, games are not different from any other software. You get many times more silent users that will never give anothing back than peple that actively contribute, and that should be fine (human nature, you can do nothing about it).
And you answered to the first part of your comment yourself. Current FLOSS games use a componentized approach. Some people develope the engines, others create the ambient art, and yet others write the stories and think of challenges. One has many opportunities to contribute in any or all of those differents aspects of a game.
As with any other aspect of FLOSS, the more people that joins the culture, the better stuff we will get.