Don't you always have to have your computer running Plex to do that, though?
I built a computer specifically for it (well, that and other things that need a ton of storage). Runs FreeNAS, six 4TB hard drives, 16GBs of RAM, etc. It has enough horsepower to send out multiple Blu-Ray quality streams to different devices without bogging down but uses so little power when it's idling that you can leave it on 24/7. It's not something you have to do if you want to go this route (I use it for more than just Plex so building your own may be a bit excessive if you only plan to serve movies/music), but I've got disks on top of disks on top of disks, so I needed something more than a computer with a hard drive or two in it to do what I wanted.
Again, it's the initial time/money sink that's the problem; once it's up and running, you're golden, but I can understand why people wouldn't bother. I would say that if you are ever looking into buying a NAS off the shelf, to build it yourself instead. You can do it cheaper and get a better, more flexible, more powerful product out of it in the end, not to mention it's always nice to you have your own personal Spotify/Netflix service to brag about.
(And if you think that's nuts, I'm planning a major expansion as I've maxed out what the enclosure can do without replacing the drives one-by-one to 10TBs or whatever. 10 Gigabit networking to my workstation, maybe switching to something clustered, probably rackmounted, etc. Going overboard is a bad habit of mine.
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Getting up, find in the disc, putting it in, waiting for it to load, skip past or forced to watch FBI warnings, finally get to the main disc menu, click that and load into the movie.
It's fucking painful and definitely takes more than five seconds.
Netflix experience can't be beat, though I don't deal with drops in quality midway like you're describing, I agree that would sour the experience for sure.
Again, barring the setup (as simple as ripping it to a PC or as complicated as I made it above), ripping it solves all of your complaints. No finding, no getting up, no loading, no warnings, just straight into the movie. And Plex even handles the extras if you structure your folders right, meaning you get the added disk value without the bullshit!
