Well, if you use websockets, you have to wire things together yourself. Sure, you might want to decide _exactly_ what data to send to each client, when to send it, how to propagate it between clients, how to deal with merging issues, etc at a message-by-message level, you want to get down to this level.

But there's probably a good chance that (unless you are doing something very specific) you'll just end up re-implementing the built in pub/sub mechanisms of meteor collections which usually give you exactly what you need with almost no work required.

To me, that's like asking "why use rails, why not just use HTTP?". We build on layers of abstraction.