Is CSS honestly that difficult? I mean I see why this could help but this just seems to add another layer of complication. I don't know maybe I'm just being a cynic with all these sorry excuses for technology.
Whether that really justifies that many preprocessors is a good question, though (Instead of m4 or sth. like that). I'm not too fond of the plethora of cutesy DSLs we've got nowadays. Reminds me of Lisp macro overuse. (Never mind the general problem that all this need for HTML/JS/CSS hackery and abstraction layers probably showas that we need a better solution altogether. As Cato the Elder said: I miss PostScript.)
Less compiles to css. It is faster to build great things than with just css. It is easier to maintain. There is almost no reason NOT to us it instead of straight css. There may be other better tools but if you have to build a lot of css, less is a great tool.