Knowledge of reactive programming changes you as a developer, makes you better and aligns your career with where the industry is going. One of the cool things about reactive programming is that the knowledge is universal across programming languages.
When AngularJS was released, people adopted it, and then version v2.0 (cough 4.0) came out and all of that accumulated framework knowledge had to be thrown away; such a waste. Interesting enough, you now need to know reactive programming to be an Angular developer - that's what the breaking change was - the Angular team has embraced reactive programming and IObservable with open arms.
If you ever decide to move away from user interface development to backend development you'll need to know reactive programming as it is the staple tool of choice with stream processing and event based architectures. Likewise, if you decide it's time to move on from the .NET ecosystem then this knowledge is directly portable and can be directly applied in that ecosystem because there's an implementation for and vibrant community in that ecosystem.
Have you ever screamed in frustration after a long debugging session related to mutable state or wondered if there was a better way to compose events when handling events? Welcome, you have stumbled upon the solution.
Internally the maintainers debate whether ReactiveUI is or is not a framework, as at its core the project is essentially a bunch of extension methods for the Reactive Extensions. With ReactiveUI, a little bit of code goes a long way and you don't need to go all-in with the framework or reactive programming. Use as little as where needed, along side an existing framework, or as your end-to-end model-view-viewmodel framework.
Buyer be warned however, the Reactive Extensions are quite addictive. Once you start writing software in this manner you won't ever want to go back. If your interested in learning a better way to develop applications please come join us in Slack. Stick around, send in a pull-request if you see an opportunity for improvement and we will invest in you, in hope that you will stick around and pay it forward when you are ready.