While they don’t guarantee good design, they don’t hurt. They aren’t a formula for success, but I’d put them in the pro’s column. They can be difficult when you are learning, but SASS makes them a lot easier to adhere to. Also, using the lobotomized owl selector helps keep the rhythm (http://alistapart.com/article/axiomatic-css-and-lobotomized-owls). He mentioned that ratios don’t work at different screens sizes, which was the weakest claim — you can just use a different modular scale at different break points if you are worried about that. I would say, vertical rhythm helps design. If it breaks, it’s like every other website that doesn’t even try, so why not try?