I can't think of a good reason :)
I looked at the whole set of slides, I think the presenter (who is also a connected to the SASS project, I think anyway?) is of the mindset to use what you know to "get 'er done"
Sass is supposedly shorter and more concise and it's possible that the speaker knows Sass inside out, but speaking from personal experience having read and written CSS for over 10 years I find SCSS much more intuitive. I do not have to learn the Sass indentation rules, and the brackets in SCSS and CSS provide visual indentation clues for nesting - which is very like most every other language uses, so anyone from a programming/coding background would be familiar with the idea of closing the nests??
.. more importantly every valid CSS is already a valid SCSS so no conversion needed to start using it - e.g. I was able to take my Drupal sheets (all 29 of them!), change their extension and recompile/compress them in very little time.. since then I've been able to take a little chunk at a time and "Sassify" it, using nesting etc. and I still like to see the brackets and semi-colons, it matters not, which you use once it's all compiled anyway!
IMHO Sass itself is too high a barrier to entry for a large project (existing one, i.e. one not built/sassified from scratch), where as it can be modularised with SCSS
So to follow the original author's (of the slideshow) thinking rather than debate pro's and cons I'll just keep using the one I know TYVM :)
