I'm not going to get involved with the: is it morally right?" portion of this debate. I DO however, feel compelled to respond concerning the legal and safety implications of some of the scenarios being suggested here. First the legal aspect. In the scenarios described above, the ride-along guy or gal manipulates the controls during the empty legs and logs SIC time.
This person has no training in the AC (Required)
Unless the certificate holder is billing the freight customer ONLY for the occupied legs. then the whole trip is considered for hire flying. If y'all think I'm wrong on this, please contact you FSDO and your insurance company. This whole "the airplane's coming home empty! You guys are part 91! You don't need to comply with :_____(wx, duty times, etc)" Is the biggest BS hoax perpetrated by less than informed operators in the industry! Jamhamms, if your passenger ground loops the -3 during rollout (and I know you would catch it first man

) your carrier's insurance company will walk away from the claim if they discover what happened. If the customer was quoted a trip that says: ABC-DEF-GHI-ABC, then it doesn't matter if the freight was only on the DEF-GHI leg. The whole trip is for hire. The operator is certainly not giving away two out of the three legs. The customer paid for them, it's for-hire.
Check your operations manual. I'm betting that it has a portion that says other than crew, the only people allowed on your freight aircraft are company employees, FAA ops inspectors, government courriers for classified material, and animal handlers. I could be wrong.
And now the Safety issues. Someone mentioned in their thread an example of a Learjet ride-along. I'm a bit of a hypocrite to say this, as I once bamboozled a Learjet flight or two back in my CFI days. (I got paid. Had to bug the the guy for 6 months, but I got paid.) While I certainly thought at the time it was a grand idea, Now I'm able to look back on it with the perspective of over 2000 hours of Lear time. The facts are this:
The Captain/Operator was a sc_mbag
If this guy had keeled over, I would probably not be writing this now.
The jet was not properly maintained. We flew three legs with a mechanical condition that I would walk away from now. As an eager young CFI, I just didn't know the difference. When I think about it now I shudder. Any Learjet Captain willing to put an un-trained person in the right seat is a miscreant. (Bring on the flames)
U-1 pilot. I feel for you dude. I remember how difficult it was to get those first multi-engine hours. If you are going to warm somebodies right seat, make sure that you recieve training first. At a minimum, some ground instruction and three bounces with you at the controls. Best of luck my friend. It seems insurmountable now, but you'll make it.
Warm Regards,