If we have something that we think the community can do for us, we will offer it. However, right now the art team is working on content that neither can be revealed to the community, nor added onto by the community. And this content is our first and most important priority. I have the bare minimum of time lately to dedicate to internal issues, and much less for outside contributions. Our time is always limited, and if I must spend a good chunk of it going over outside contributions that don't meet our quality standards, I'm wasting that time. I'd much rather use what I have to finish what I know we need first.
Regardless, people are always welcome to discuss contributions with me and suggest a model they would like to create. If we can put it to use and if I know the artist is capable, I will say so and help him/her out with modeling, texturing, etc. I have done this on numerous occasions. (Terrain and level design however is out of the question - too much depends on it to be handled outside the team, like connection to previous maps, assets, settings material, etc.)