Q3A's basic modes of play might as well have been a tech demo. So before I ramble too much, let me say this. However, I feel it is important for the game companies to realize that it is their JOB to put out a successful GAME, not just release an SDK and engine to the community and let it sail. You raise a valid point. But keep in mind that nobody is going to make mods for a crappy game.
People do that in order to continue a great game they enjoyed so much.
Whilst Q3A isn't exactly the most sophisticated of gameplays, it is certainly the best straight-deathmatch game out there. Especially to the die hard game fans out there. I mean there is so many different mods out there, that they will always have that different feel to them that will have the gamers comming back for more. As for what the developers should do is encourage it, keep releasing the source code so that people like us can do the mods that will keep the people playing, and always changing the way the game is played. I left PlanetAvP myself because I was so upset with the situation.