Many people do because they are easier to walk in...including me. I would also like to know what I am missing. Perhaps I pull around a few extra grams. Perhaps I lose one second on the start-up. Is there anything that matters to a B rider?

"""...to avoid buying a new pair of pedals and shoes.""

Your MTB pedals will work fine. But swapping the pedals back and forth between bikes is going to be a pain. You can get MTB pedals cheap on eBay.

""Chainwheel"""

The only problem is that your position on the pedals may feel less precise. Fit on an MTB doesn't have to be so perfect, but on a road bike, it's another story.

I use Crank Bros. Candy pedals with Sidi shoes on one of my road bikes. I've been to enough races where racers are using them or even Eggbeater pedals, because they are so light and they work fine.

I'm also using some very stiff carbon-soled shoes with Time Impact (road) pedals on another bike, and I can tell you, the interface is perfect. Lateral movement is adjustable but it almost feels like my foot is part of the crankset. Of course, now that I'm racing, I'm spending a lot more time practicing sprinting up steep hills, and as a B rider, you may never do that.

You might actually get the same effect by using a carbon-soled mtb shoe, but I haven't tried it.