645's are (were) for wedding photographers. That's all the redeeming quality I can see in one. You can't buy 220 any more, so it can't replace a 35 for shooting willy-nilly. And it can't do what a 67 can do in terms of neg quality. It's closer to a 35 than anything else, so why not stick with a 35? Just extra bulk and expense for diminishing returns.
There have been a couple of 645 threads recently, and I've seen this comment come up a number of times - the 645 was a wedding photographer format. I don't know if it is written with derision, but I always seem to read it as derogatory.
When I think about it, a wedding photographer requires a number of difficult compromises:
-The camera has to be light, because they will be carrying it all day
-The camera and lens system has to take good shot in all sorts of conditions (dark churches, bright open areas, near, close, etc)
-It has to be quick to handle, since your subjects won't be holding still much, and there are no do-overs for missed shots
-The negative has to produce fantastic 8x10s, and will be expected to make larger enlargements from hand held shots.
I mean, when's the last time you read "Monorail 4x5s are for studio work. That's the only redeeming quality I can see in one." ?
Okay, so you get 15 shots instead of 36, so you won't be shooting willy-nilly (although multiple backs lets you shoot willy-nilly and in multiple emulsions if you so chose).
And yes, the negative has half the information of a 67, but that requires you to have a bigger enlarger to make use of it. I'd say it's closer to 6x6 than any other format (6x6 only has 1/3rd more surface area, instead of the 3x the surface area a 645 has over a 35mm).
It doesn't sound so bad to me, but then, my favourite colour film is Porta 400, which if I understand it, was created specifically for the wedding market as well.