@adamwathan thoughts on so-called 'silent' extends in scss, ie, the whole '%class-name' thing? Could that work as a sufficient substitute for the LESS class mixins?
The big issue with extends in my experience is that it changes your source order which can cause unexpected bugs.
I'd stick with regular Sass mixins, and if you want to mix in an existing utility, extract a mixin from that utility and mix it into both the component and the utility.