Decks
Whether or not you prefer the fuel or the co-pilot for IFR ops must be influenced by the type of aircraft and how it is being operated. On an S76 90 kgs clearly doesn't buy you much extra time and maybe you're operating well below max wt for IFR ops anyway, so it's irrelevant. But if you're not, and operating say an IFR EC135, AS355F1, A109A/C, for example, that extra 30 mins of fuel can be a more valuable safety net. Furthermore it's going to allow you to go further, be at the scene longer, and have a greater choice of alternates if you have to go IMC. If the terrain is more demanding and weather bad, co-pilot benefit increases, agree.
And then in the real world there is the extra cost of a co-pilot to consider. Safety comes at a price, and that price has a limit, but that is another whole new subject!