Using steamworkshop probably won't affect third party websites like the Nexus. The actual content created by users for Team Fortress 2 is optional, you can unlock it through time or crafting, or pay to get it right away. The map content created as well as is, basically, if like the map and want to support the person who created it, you can buy a stamp with the money going right to them. If they added something optional though (they said it would be free) I wouldn't have much of a problem with it. For the
larger well made mods I probably would donate something. Keeping track of it as well would probably help the developer get an idea of what users like and maybe even allow amatures to move into professional development. That's how Valve itself started, most of the staff were modders. Team Fortress was a mod. Portal was a much smaller game created by students subsequently brought over for AAA treatment.