Don't leave the rear brake on. Bad idea. A rider's feet are directly connected to that rear wheel. You slam on your rear brake, guess what stops turning besides the wheel? Recipe for flying, or at least grinding up some part of your lower leg flesh, depending on your pedal clip combination. A front brake is sufficient, provided it's in good working order.

As regards the crank with an integrated chainring/crankarm, I don't see how that would necessarily cause you any problems. Maybe your critic is just of the mind that such a crank is lower quality--which it is. But you could probably have similar problems if you were using any cheapo crank/chainring combination, regardless of whether the two are connected via a spider. Just tell people the big ring's basically a barbed bashguard .

PS Might want to post a pic to give us a better idea of what this looks like.