Kalypso Media has not supported user-made mods in previous versions of Tropico. Many users have pointed out that this prevented user generated solutions to common Tropico problems such as traffic jams at 4-way stops.
The Tropico community is low key, but players have put a lot of effort into designing work around solutions to common tropico problems. FritoPatata created and in depth guide for a transportation strategy. As much as I appreciate the ingenuity of players like Frito, I can't help but think the lack of mods is creating a game in which their is a limited number of ways to win. Once you've figured out a strategy in a game, it is hard to break from what works and try new an innovative ways to win a scenario.
By allowing players to create mods and list them in the steam workshop, Kalypso would be improving the long term playability of the game. Kalypso could be worried that allowing players to build and share mods in the steam workshop would detract from their ability to sell Kalypso created DLC. But I think a healthy community of players can support both user created mods and Kalypso created DLC.