Yes, it's certainly something on our list of things to address.
We do actually have support for varying the number of workers per website on the backend, so there is a (slightly messy) workaround that might be of use for you; let's say you needed 24 workers spread over three websites (20 for production, 2 each for staging and dev). You could set your account up so that you have 8 workers per website, for 24 total, and then email us and we could re-distribute the workers so that they were allocated to your websites in the pattern you wanted.
But obviously, a proper user interface for that would be better, and it's something we're planning to do -- no timeline at present, though.