Not to be a pedant, but did you mean "guides" in the title?
Studying the features of proprietary products is not in itself bad at all, and (as you mentioned) can be a great way to gain ideas for free software equivalents. If you're paying for the guide with money or your personal data (or both), then there is a little harm being done, but it's probably not worth worrying about it too much. Avoiding those payments isn't usually too hard anyway, although it might be illegal.
