I've stumbled across this problem in my current project as well.
A method in world or entity manager would be nice, as having to write a system for every component just for getting all entities isn't very nice, i'd say.
It would be a quite expensive to gather all entites with said component. You would have to iterate through all entities and check if they intersect with a bit set that only contains that one components index.
I think I'll add this, but it really shouldn't be the way get entities, systems are still the way to go!
Take a look at this fork down here in junkdogs signature (or in mine

).