The analogy is helpful, but leaves some things out. In one of the grocery stores, you'll find that if the lights flicker off and for just a second, half the products on the shelves may end up unusable. And if your store gets past a certain size, a sinkhole may appear and suck most of your inventory underground with no warning. The only reason I can think of for using MyISAM tables in a web app is the terrible default…
Even after you've spent days tweaking the performance, you are still left with a fundamentally slower engine.
I'd rather have a consistent database than a fast one!