And here's the argument against them: the walls of a mosaic of images is the thing to do these days, and my matted 5x7s sell well at shows because of this.
The cards here are 5x7 and fit the same purpose - people often will mat and frame them, they are after all only an inch smaller than your smallest print and should be priced accordingly. If you go to a mixed box, they can get a box of 10 for a couple bucks a piece or whatever your lower markup is on that. I'd be out of business fast if I sold 10 5x7s for $20 profit, when I could have made $150 on those same 5x7s selling them individually.
If the cards were more of a notecard size... 4.5x5 or so then I could see doing mixed lots.