As others have said, it's certainly justifiable for anything with rewatchability. You can find 1080p Bluray players at thrift stores for under $20 (it took some patience for me to find a relatively newer one that was more compact and didn't have a slow interface), and Blurays are usually under $5.

I've come across sealed Blurays or even the occasional former Blockbuster rental so it's actually kind of fun in a record-crate-diving sort of way, if you're into that. But of course you can always just buy new, which you'll probably have to do in some cases: the odds are against me finding Koyaanisqatsi at the thrift, let alone the Criterion Collection Qatsi Trilogy, but I'm okay with that.