I use preprocessors in pretty much all of my projects, however, there are two important reasons not to use a preprocessor:
When you're learning CSS - This is the most important. If you don't know CSS, don't use a preprocessor. Period. Move on. This falls into the same category as learning JQuery instead of learning JavaScript. I love CSS preprocessors, but they can obfuscate some very important aspects of CSS in favor of speed and ease of use.
I used to have the same problem with colleagues and friends talking about CoffeeScript like it was the second coming and then realizing that they didn't actually know what it was doing in the background and how to write the equivalent functions in Vanilla JS.
If you know CSS, by all means LESS and SASS it up; if not, crack a book :)
Sometimes you just don't need one - There are plenty of times when I'm working on a simple site or just stubbing out something really quick that it'd actually take more time to get your environment up and running than just writing some CSS in notepad. Sometimes simplest is best.