Simple use of the POSIWID principle can lead to gross error.
Many users of the POSIWID principle seem to assume that system scope and behaviour is unproblematic - that outcomes can be unambiguously attributed to some system or other.
A complex system apparently produces a diverse set of outcomes. According to the POSIWID principle, all these outcomes may be identified as purposes. A complex system typically contains many conflicting or contradictory purposes.
Many of the purposes of a system cannot be observed in its normal behaviour because they are contingent purposes - the behaviours may only appear in extreme circumstances.
Furthermore, some behaviours are generated by the interference between many systems. It is of course possible to frame the interference within a larger system, whose purpose is presumably to generate this interference and its effects, but this framing is not generally a useful one.