Actually, you shouldn't discount the potential usefulness of running 
apply() in all of the worker nodes.  A lot of people coming from 
parallel programming know about things like global broadcasts, 
reductions, and so forth.  For example, if I wanted to perform a global 
operation (maybe some kind of configuration) on all workers, I could see 
doing some kind of global apply() operation to do it.  

That said, I'm not actually asking for any new functionality.   I'd just 
make it more clear that apply() is not performing a function call on all 
pool workers.

Also, given that apply() blocks, I'm not exactly sure how useful it is 
in the context of actually performing work in parallel.   You might want 
to emphasize that apply_async() is better suited for that (the only 
other way I could think of to take advantage of apply() in parallel 
would be to call it from separate threads in the process that created 
the pool).