Not important anymore. Example:
When BF1942 was around, there were dozens of mods worth playing within a short time after it's release. And they were POPULAR. I mean, Desert Combat, Forgotten Hope just to name the two biggest (I think).
Now, in the age of Battlefield 3, there is no real prospect of a mod coming out for it any time soon. And the mods for BF2 weren't there until much longer after the initial game release.
This is because of two things. EA/Dice isn't as friendly to mod teams as it used to be, but much more so, and this goes for ALL games, they really get more complicated. Models take more time to create, physics, lighting, everything is bigger (and better) then it was. So it is much harder for mod teams to complete a rich filled release before the game's population has declined and moved on to the next thing.