id-softwares primary target is to earn money. and they dont earn the money with the game itself, they earn it with the technology behind it. that allready happend with q3, think about how much more the q3 was able to visualize than shown in q3. thats the same with the doom3-engine, doom3 and quake4 are techdemos, high-professional tech-demos even the q4 singleplayer.
but idsoftware's also supports the modding-community a lot, just check out the devellopers-center (iddev.net or sth like that), great source for modders. but thats just one more marketing strategy to build a rich community around q4 and this will help developers a lot. q4 has good chances to get as fam. as q3 is.

greetz yeah