Originally Posted by DBono

Correct! We were talking about LOGGING PIC time, you were talking about acting as PIC. Pilots can definitely log PIC time even in situations where they can't serve as PIC.

61.51
(e) Logging pilot-in-command flight time. (1) A sport, recreational, private, commercial, or airline transport pilot may log pilot in command flight time for flights-
(i) Except when logging flight time under §61.159(c), when the pilot is the sole manipulator of the controls of an aircraft for which the pilot is rated, or has sport pilot privileges for that category and class of aircraft, if the aircraft class rating is appropriate;

This is clearly the common situation where a PT 121 SIC is PF, sole manipulator. Since 121 has its own requirements, such logged PIC time clearly only has value in 91/135 situations.

The 61.159c exception applies to cases where 91/135 operators have written opsecs to allow co-pilots to log SIC time in airplanes that are not certificated to require a co-pilot. Not applicable to Pt121.