At this point in time there isn't an open sourced mod project that is both finished and represents a good example of what spring is capable of. I'd agree though, once there is one that would be far preferable to including with the game then a mod with content that doesn't belong to us.
Well to me spring is one of the most impressive open source games ever. I haven't really played it very much, but I'm still at ~3600 in-game minutes which is way more play time than I get from many commercial games.