About swapping out the backs. First, I would rather not have to buy extra backs. They're quite cheap, yes, but, many of one adds up. Seems like there are a few people on here who have ten lenses, five bodies, and all the trimmings--but that aint me.
Second, it certainly is fast to swap out the backs, but they have to be loaded sooner or later. And yes, 20 seconds IS a long time for me--as I suspect it is for many. It's not that I'm impatient, per se (I'll wait hours to get a shot), it's just that this is one area where I would rather be doing something else.
Third, those Hassy backs are bulky as all hell. I like to roll with just one camera, one lens.