I think this is a great idea, it basically means making a very good product better.
So why are User/activity triggered notifications interesting to me as a radar user?
I love great UX (definitely includes notifications). I want to be told what has happened while away especially the threads that have set the house on fire a la Uber killing our local startups
We all love simplicity too but simplicity with excitement? Some users will find it interesting
You can always turn notifications off in you profile settings. You can change the frequency of the notifications as well. Loads of applications currently have this as a feature.
API is an interesting angle. it would be nice to see what else can be done with/on Radar. But for this notification feature, the TC guys can easily do this. They have (own) the data! I am sure they don’t need a public API for third-party app integration to sort out notifications