@seabird20 writes that "POSIWID should be plural..Systems "do" many things. Expense system purpose pay employees or control costs? Whose perspective?"
Sometimes that's exactly the problem - we expect a single system to do two different things at the same time - not only paying employees but also controlling costs. The employees may complain because the system is slow and complicated, and for really small claims they sometimes lose the receipts or can't be bothered to claim at all - but of course these effects contribute (whether deliberately or otherwise) to the "controlling costs" purpose.
The POSIWID principle at least helps employees to understand why the system behaves the way it does, they can work backwards from the manifest consequences of the system, to infer its purpose.
Now let me come back to Chris's point about plurality. How many purposes do we really have here? Can we regard paying-employees-and-controlling-costs as a single complex purpose (with some internal tension), or should we regard it as two different purposes yoked together (rather like life-liberty-and-the-pursuit-of-happiness)? There isn't a definite answer to that question; we just can't count purposes the way we can count cans of baked beans.
And we don't always know how to count systems either. Is there only one system here with many purposes, or do we have many "systems" occupying the same space? Perhaps the effects experienced by employees are the result of an interaction between two conflicting systems.
In general, however, there are many systems and many purposes. So POSIWID helps us identify additional purposes that explain the perverse behaviour of complex systems.
Related Post: VPEC-T and pluralism (June 2010)