Here's another perspective: I have worked as DO with several 135 operations and in all cases, an SIC was required for any IFR operation unless the aircraft had a functioning autopilot, and the PIC had successfully demonstrated on his FAA check ride that he was capable of using the A/P "in lieu of an SIC" and that authorization was noted in his record. The POI also informed me that even then the PIC had the option of using the A/P in lieu of an SIC or using an SIC. The SIC also needs the check ride with the FAA and appropriate ratings. One new startup that I worked , drafting the manuals etc., on which I was to be the DO, (I no longer had any desire to be a charter pilot), I took the SIC ride so that operations could be conducted when A/P was inop. Unfortunately on the ride, PIC in left seat, me in right acting as SIC, PIO behind us, the PIC was busting an NDB approach, I reached over and pointed to ADF needle to warn the PIC; a little later on a single-engine circle-to-land he was about to bust an altitude, and I tapped on the altimeter. In short - he passed his check ride but the POI said he would've busted it if he hadn't had an SIC and wouldn't give the A/P in lieu authorization.
Reuslt: I had to ride along until his next check ride, and I did log the SIC time as well as PIC for time "manipulating'.