I suppose they also have caching built in, but that is easy to build on top of FRP.
"One question you may ask yourself, is why RxJS? What about Promises? Promises are good for solving asynchronous operations such as querying a service with an XMLHttpRequest, where the expected behavior is one value and then completion. The Reactive Extensions for JavaScript unifies both the world of Promises, callbacks as well as evented data such as DOM Input, Web Workers, Web Sockets. Once we have unified these concepts, this enables rich composition."
When people are trying to do fancy streaming stuff with futures and come to me for help, I generally recommend they do the streaming with Rx instead. (eg rxcpp from Microsoft)