The designers produce the CSS, I don't. In most cases, they use their fancy tools to produce it. If they're not using SASS, what's the advantage to me? Why would I convert their CSS to SASS to get the same result in the end?

Templates are a different story -- that's where the design and the functionality meet. I do convert the final comps to templates and if necessary the templates back into static code to repeat the process (rarely necessary, but it happens). This is a necessary evil. Why would I want to perpetuate that evil in CSS as well?