We recently had to fire a design team and are in the process of moving to a different team. If all the styles were in SASS, I'd have a much bigger problem.
Even something like LessCSS has a much smaller learning curve because it looks like CSS. You can use it much more minimally (using only variables, for example) and get a small benefit with a small learning curve. Think of how many of those 50 other languages look like C.