Which is how I ended up playing it -- you can play a full planetary war in under an hour, so we were able to do a real campaign during our lunchbreaks.I think may also be a perception issue with the game. The ultra-light rules and non-prep setup lend themselves more to emergency pick up game. However the word is, from those in the know, is that 3:16 only really come into its own in an extended campaign. At certainly, if you look at the rules id clear that certain dynamics and rules only come into play further down the line. But I suspect the pick up game aspect is probably what gets more actual play time.
That was my main worry. I actually appreciate light rules as opposed to the massive compendia that come with D&D/Pathfinder, but the creature descriptions feel like I'd need to put a good bit of thought and effort in to make them come alive as actual aliens that the PCs are slaughtering instead of "monster movie cliche #6". There's also the issue that some Alien Special Abilities appear to be nearly identical (Leaping vs Fast Movement, Armor vs Ignore Wounds).It's very bare bones in what it generates for opposition, so be prepared to think a bit about how to describe creatures and also how to include lot of setting so it wont just be "so you fire your guns, they take hits".
But in general it's a great game, great fun. I have played it a lot and now I feel the urge again.