you cant log the time
And how DO you log single engine F/O time
can you even log Co-pilot time in a single?? I know my logbook doesn't have that column
Of course you can log it - If you are appointed by the company as co-pilot for the operation, then that's what you log. As to their reasons for multi-crew single-engine ops ? Well that's up to them and (usually) the customer's requirements.
As for not having a column in your logbook, there isn't one for Instructing time either, but CASA very clearly require that to be logged. It's hardly rocket-science; Either use one of the spare columns, or simply re-name one of the columns that you don't need.
http://www.casa.gov.au/fcl/flight_time.htm
You can choose for yourself whether the logbook time is useful to your own flying ambitions, but please stop presenting wildly inaccurate guesses as facts.