Having a workflow in multiple states is one of those features that makes things much more complicated. In fact one of the reasons I created this was because I was exposed to Open Symphony Workflow through JIRA, both of which are Java products. OS Workflow can do the split/join mechanism but IMO it makes the configuration much more convoluted, and this complexity was something I really wanted to avoid.
So my tack was to not include it in the beginning and see what people thought, adding it in later if demand warrants and if we can add it without making things too complicated. I think you're right that it's really useful.
I really dig the idea of a GraphViz view of a workflow. I'll see about that.
Also, this isn't tied to SPOPS at all. (No pun intended...) SPOPS is one of the persistence mechanisms, but you don't have to use it. There are also a generic DBI and a file-based persistence stores as well. If I used Class::DBI there'd probably be one of those, too :-)