Said methods are only economical when they actually work... which they often do, or no one would espouse their use. But when they *don't* work, when you end up back tracking and starting over from scratch... the same 'cost' in time and effort could have been better spent up front with a somewhat more rigorous testing method.
Some people have great fortune with abbreviated test methods. Kudos for them. Me, not so much. I've been 'lead astray' too many times to put much faith in doing things that way. But if it works for you, rock on