Actually, XLS is not bad to work with, if you have a library for it. And it's well defined, unlike CSV. Insofar as I know, there's no way to make a CSV file that will open and show nicely in all popular versions of excel / google docs/ open office, especially across language settings. And a well formed XLS file will just work.
I use CSV all the time when I am working with R. My data can come in the form of CSV, XLS, or PDF. Which would you want to work with?
I can easily look at the data. I never touch my incoming data and my output is in reports, but CSV can be the easiest way to get data into a computer.