1. Steam Workshop: User-friendly and very easy way to share your creations and check other peoples stuff out.
2. Steam Library: no need for tons of launchers/applications running.
3. Steam Community: Chat with your friends while in-game, without tabbing out.
4. Steam Community: If there is ever a multiplayer, Steam provides a very easy way to create own Servers, join friends and invite friends.
5. Steam Community: Easy and fast way to share your Screenshots and Videos and recieve comments on them.
6. Steam Community: Built in guides, forums and whatnot, which are easy acessible without having to tab out.
7. Steam Library: No need to redownload the Launcher and files every time.
8. Steam Cloud: Savegames are safe.
9. Steam Updates: Updates through Steam are easy and fast. Also Update news are always nice to have straight away, without having to check forums.
That's all i can think of for now. Steam has no contra's for users. The only downside is that the Developers don't get 100% of the money.
I think the fact that you would be forced to use Steam if the user generated content was only able to be shared that way is a pretty big downside.
Also, from a development point of view, having Steam dependencies in your application might not be ideal.
When it comes to the money aspect, I'd rather see Frontier invest more into developers and artists to improve their game, than feeding money to a third party.
I never used any of the Steam stuff you mentioned. But I liked the workshop integration for Cities Skylines. However, after a while, it got quite messy, with lots of broken or abandoned assets (due to game updates), which made it almost impossible to find the good stuff.
When relying on Steam for example, I don't think Frontier could update player's assets themselves, when they decide to introduce a breaking change.
When Cities Skylines introduced nighttime, that would break pretty much every UGC there was on the workshop. If the assets where under the game developers control, there was at least a chance, this would not happen.
I think in the end it is all about usabilty. If Frontier can make something better on their own, I would definitely encourage that. The fact that Steam is already there and used by many people, is not an argument.