And the best part is that while the key is valid, the license isn't, so technically you're paying for a pirated app
(Explanation: those keys come from msdn or from a Microsoft partner action pack - they activate and look legit, but are licensed only for development and internal testing for that company)
I've seen office professional for 15 bucks, I wouldn't say $25 for the basic version is a good deal… Especially considering you end up with a Microsoft product.
The idea is that even if you're using pirates windows, you're still in the windows ecosystem, you'll be more likely to pay Microsoft for other things. So in a weird way, yeah you didn't pay for windows, but you also didn't buy an Apple either, and now you're using windows apps and programs.