Paul,
I fly a Cessna 414 and Kingair B100, both certified for single-pilot operations. Sometimes we fly single pilot, sometimes we don't. It all depends on what the customer wants, or what his insurance company dictates. For example, when we're flying the officers of one of the local banks, the bank's insurance company requires that there be two pilots on board. Other people just prefer that there are two pilots, it makes them feel safer. Additionally, sometimes it isn't passenger's insurance company, it's the operator's insurance company. Many aircraft insurance companies are now requiring that aircraft such as Kingairs are operated with 2 pilots, especially when used for charter.
As for logging SIC the regulations only permit logging SIC when required by the type certificate of the aircraft, or the operations under which the flight is conducted. For example, an aircraft operated under Part 135, that has ten seats or more, MUST have two pilots, per the regulations. If you were flying a Kingair 350, which is a single-pilot airplane, on charter, you would be allowed to log SIC as co-pilot because the regulations require it for that operation.
Other than that you may not log SIC.
Ray