I can't do any kind of IO in a webworker, which means things like indexing the filesystem are impossible there. The vast majority of cases where I need to do background work I also need access to the host.
EDIT: I should note that I did start with webworkers for things like auto-complete, but what I'm doing now (spinning up another instance of node) is a lot more powerful and robust. As scary and sad as that may be.