e.g. I'm not sure how I'd approach form validation if you used only coroutines. In one of our forms, we do a lookup of city and state based on a zipcode field and country field. So on any changes to those fields, you want to do a REST API call to see if you can look up a city state, but you need to throttle the number of REST calls so they don't happen more than ever second or the REST API thinks you're doing a DDOS attack. This is almost trivial to do in Rx...