The HBO subscription is only worth it if you have a peer group that also has an HBO subscription and so it's important to watch things at the same time as them. I stopped buying DVDs about 10 years ago when renting became a lot cheaper than buying, but I've recently started again with boxed sets. Even if I only watch each episode once, it's cheaper than any of the streaming options, plus they're practically DRM free (as in, the DRM is so broken that it may as well not exist) and I can copy them to a mobile device for watching on long trips. Oh, and I get to wait until there are multiple years of something before I watch it.
I do wonder a bit what would happen to the economics of TV series production if most people did this. You'd expect a TV show to make a loss for the first few years, but then be profitable over a longer time, which is a very different model from the current mode of any profits after the first year are a nice bonus, but not factored into the accounting calculations.