I too had to struggle with justifying cost of a Horseman back, nothing around second hand at the time I was looking, they are still quite scarce. Because I love the porportions, I took the plunge. Although I have subsequently found a reluctance from editors to use this format as it does not lend itself to usage for cover or centre spread images.
Agree in theory that can crop from 5x4 and be flexible with where the crop should be. Personally I find this hard to do, usually making a poor 5x4 shot, as I seemingly have an inability to frame to arbitrary lines on the GG. So making the decision to go 6x12 for an image ensures that I then compose tightly into the mask which I made to go over the GG. This mask was made from plastic card from a modelling shop rather than fork out £20 for a set of Horseman ones.
In all honesty, I have used the back rather less than I expected but still lug it around.
Concur Tom's points on economics. Can bracket exposure and try different filtration, all on one roll. Only hassle is if do 3 frames and then want to switch from B+W to colour or vice versa. Carrying two backs is not a solution!