I don't get it either. I have some 220 because I got it in a box of supposedly frozen film I bought off the 'bay, and the film all works perfectly, so I bought a 220 insert for my 645 for $15. For that price it was worth it for the occasional roll I might run into - works fine in the same backs, plus I can shoot it in my Yashicamat 124.
If B&W films I liked were available in 220 for no more than twice the price of 120 I'd use it - some. But to my mind one of the advantages of medium format anyway is that I'm not stuck with 30+ shots on a roll and take five or ten of some subject then have a roll of film totally unsuited to the light/subject/intent still in the camera with 2/3s of it to go next time I go out photographing. The 12 shots from my Yashicamat are about perfect. Sometimes the 15 from my Mamiya are already too many. This applies mainly to film slower than, say, 400. If I could get Tri-X or HP5+ or Delta 400 or TMY in 220 I'd probably shoot that in the Yaschiamat because I use it 90%+ for black and white anyway.
But pay way more per shot? No way. They might as well just dig the hole and push 220 the rest of the way in before I'll do that.