Plex has far more and far more useful channels. Even Netflix is better on Plex (and face it, Apple TV is basically a glorified Netflix box, unless you are stupid enough (and I suppose, a technical imbecile) to be spending your unemployment checks on overpriced media from iTunes).
I am not sure of your level of understanding of iTunes and Apple TV that's causing you to make such an irrational statement.
You are aware that Apple TV can stream both movies and audio that are not purchased from the iTunes store?
Assuming you know that, you know that having stored local media costs money and time in infrastructure and that you have to take that into account when comparing costs of purchasing from the cloud?
You also realize that "properly" storing local media is no simple task no matter what software solution you use. When you start getting in the 10's of terabytes you have to start being more methodical about how you will account for several different scenarios that could cause data loss.
Some people may be able to download all their movies and music for free from the Internet and then steal USB drives from their job at Fry's Electronic. But just because that guy can claim that having a movie collection of 2000 films costs nothing, it's not a solution for every one.
There is also content that you cannot buy on physical disc and is only available through digital content services like iTunes or Amazon. If you want to download them from torrent sites for free that is a personal choice, but it is not an option for all people.
The unemployment check aspect is really beyond the scope of my knowledge.
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Since plex cannot stream iTunes movies it is not needed unless you are copying your discs to your pc.
To clarify, iTunes can handle copied movies just as easily as purchased content and their is no distinction between the 2 on iTunes and Apple TV.
The caveat is that you will need an external metadata tagging program to get the same "look & feel" with your copied movies. Primarily if you want things like the "HD" logo and the rating to show up.