This ... is the feature I need, and didn't even know I needed.
There have certainly been times where I've wanted to cap the number of active promises, to prevent sending out a gazillion REST requests or database queries at the same time. Sometimes this occurs in production code, and the amount of ugly (and potentially buggy) code I would have to put in to implement this properly is scary. Sometimes this occurs in a database-upgrade script where I'm not really inclined to do anything too complicated or depend on anything external, so I just throw together a very non-performant solution.
This also provides a very easy solution for the whole sync-errors while setting up promises issues, as discussed at length here (it might not be what the O.P. wanted in that thread, but it'll certainly make me happy).
It's a very creative API too - I love how simple it feels, yet it has so much power.