I don't see any problem with pilots building time flying FAR Part 135 on-demand stuff. It builds character and good know-how because at most 135 operators the pilot has to do everything themselves. Also flying to random airports with little or no notice keeps you proficient at reading charts, adhering to procedures, and maintaining superior situational awareness.
However I don't think flying as an instructor creates quality pilots. I've found in many cases CFIs that are hired at my company struggle a great deal more during training than pilots coming from other backgrounds. Our check airmen and I have theorized about this and think it could be because as a CFI or CFII the instructor isn't building any actual instrument time. This coupled with the over-familiarity many instructors have with their home base, their lack of actual "stick" time, and the fact that training is usually conducted in fair weather makes for weak skills.
I think flight instructing is a good stepping stone into the 135 world, but from an airline perspective I think there should be a step between instructing and flying for a scheduled carrier.
That's just my two cents, though.