If you don't offer mod handling in your own game then using Steam Workshop is the only userfriendly way of dealing with mods. It's such a pain in the butt to stay up to date if you use mods that get updated frequently (new tire models, new shaders, new physics in game updates, just general updates to the mods...) if you have to download and install everything again once it's being updated.
I know for many of you this might be a non-issue, but it actually keeps me from installing any mods in AC, especially when those mods would come from several sources...