I'd offer to compare the cost of "this hobby" to buying a boat...
The two best days of boat owners famously are the day they buy it, and the day they sell it
with photography, as much as I love buying gear, for me the best days are the hundreds of days after buying, the days when I am out in the field with whatever camera I have with me.
Back on topic, I think there is room for 6x12 backs in a 4x5" shooter's kit. Also, carrying a 6x7 (mini-4x5) film back and a 6x12 one would weigh as much as 6 4x5 holders, but one could carry a few rolls and have many more shots, not to mention the ability of changing rolls in the field, as others pointed out. The availability of extra emulsions in 120, with some super-sharp ones that aren't available in 4x5", should help compensate at least to a degree for the lack of real estate in 120 compared to 4x5".
As well, using 6x7, 6x9 and 6x12 on a 4x5" camera body and ground glass allows one to "see" outside of the actual frame recorded, which is great for precise composition (especially with cut-corner 4x5" ground glass) and for precise focusing, since it allows one to use a loupe right to the corner (and past), whereas with 4x5 the frame around the ground glass makes doing so much harder.
Last, if one needs to fly, it seems that carrying roll film makes it easier to have it hand-inspected compared to 4x5", which might be a plus as well - I know it might be for me, which is why I am considering adding a 35mm lens to my 4x5" kit (to replace the 47mm and 55mm I am using with 4x5"") and travel with roll film on my fall tour... better having less real estate, than having my film nuked my a CT scan machine!
But, the beauty of a large 4x5" sheet is of course something else...
Best regards,
Vieri