The problem usually arises when the GC pressure introduced from the promises (often the intermediate promises) created by many active/concurrent tasks starts burning copious amounts of cycles.
As your concurrency increases, a poor promise implementation starts burning copious amount of valuable cycles, often largely due to GC pressure.
In the abstract this doesn't sound that bad, but when comparing well behaved implementations such as BlueBird, RSVP, When, ES6-Promise etc. with the current state, (July 3, 2015) native Promises, the difference is still staggering.
As for the browser, a promise is a great way to handle async and often a great abstraction around a single potentially remote entity. As more ambitious applications are created, it isn't uncommon to have thousands or tens of thousands of these remote entities. Wouldn't it be nice, if the overhead of using the promise was negligible?
