Garry's Mod is a game with comparable modding capabilities that has this feature, including the option to specify an alternate download server.

Unsurprisingly, there is much more variety on public servers, not just a few standard maps everyone is tired of.

Edit: This is a duplicate of issue 0006428 and related to 0009111. I used the search function but only found these later through Google which apparently handles similar keywords better.

Malicious software can only cause harm if it is executed, a mere download does not cause any damage.

This is nothing new, it has already been implemented in many popular games several years ago without viruses spreading although anyone can host the content.

Nowhere in the description does it say download from the game server. Using Steam Workshop or whatever free third party hosting admins may find there would be no increase in bandwidth. This is a very reasonable idea and pretty much overdue in 2013.

Also this could surely be made optional like in other games.

Here's a nice example of a smooth Steam Workshop integration: http://garry.tv/2013/07/04/server-downloads/

Check out the video. A single click on join and it handles the rest. Note the 3 second map download and installation. That alone would bring much more variety to public servers.