Whilst I agree that generally a cabin attendant might not add a huge amount to the overall safety, I can say from experience that a good rear crewman is worth their weight in gold when required. The problem here is that the times when a crewman is really required for normal civilian offshore ops is so seldom that nobody really is going to justify the cost. Of course it would be a very nice to have in many situations, and emergency handling in a 3 crew environment can be far slicker than in a 2 crew, if trained correctly. But here we are talking about a nice to have not an absolute necessity. In the cougar aircraft a rearcrew could have helped by looking out of the window and confirming the oil coming out of the side, and could have assisted with the checklist, maybe even offered a "casting vote" on the land immediately situation the crew were in and in slight disagreement about - however the last is purely speculation and it could have been easily the other way. A crewman would also be very nice when landing on the platform especially at night and in bad weather, giving additional guidance and clearance, but we have been doing it for years without one so difficult to suddenly justify. All of this however could also be done much more easily and cheaper, and it amazes me that today we still have not come fully on board. The use of a camera on the tail boom can certainly go some way to giving more vital info to the crew. Whilst some aircraft already have these fitted, it is far from standard in the industry. Even my car has one to stop the wife from hitting the lamp post! (Sorry wife!). Even the 332 L had mirrors fitted to allow the pilot to have a look behind at the top of the aircraft.
In the cougar case the crew would likely have been able to see something awry if they had a view forward over the fuselage. This is also an extremely nice feature to have in cases of engine fires, various oil leaks and other things that can go wrong. In addition it greatly enhance safety when landing during normal ops.
A more relevant question to raise might be why are externally mounted aircraft monitoring cameras not installed as standard, as opposed to why are we not carrying a cabin attendant.