It's funny that everyone here is expecting airtight security practices, proper vulnerability disclosure and general trustworthiness from an app that's a tiny step above the likes of BitTorrent and PopcornTime.
With most BitTorrent clients you don't have a remote account on the creator's centralized system so there is nothing that can be compromised in this way. If software devlopers require or strongly encourage online integration then yes they should be held to higher standards.
I use torrents frequently for downloading Linux distros. Last weekend I downloaded Debian[0].
Plex has many legitimate use cases. More recently they've pushed into streaming by licensing free content[1], and adding a UI to show where to watch a given show given a set of streaming services that you subscribe to[2].
Aside from that they have their streaming Live TV/DVR[3] feature where you plug a TV tuner into your computer to stream your TV from anywhere, and record it to disk. I have this setup, and it can be useful when watching sports with friends because nobody I know has cable setup 24/7 -- they usually plug an antenna in when they want to watch a game.
The more common Plex features of managing a TV, movie, and music library have legitimate use cases of providing a way to stream media that you have purchased and copied to disk.
Lastly, just yesterday I uploaded all of the lecture videos of a course I'm taking to my Plex instance which allows me to stream them from anywhere or download them for offline viewing using a reliable app instead of the Canvas UI.
Out of honesty I'll admit that I do use Plex for piracy. I previously had subscriptions to Netflix and HBO, and I'll often purchase a subscription when there's a new show that I want to watch (e.g. the new Game of Thrones series). I'm not going to subscribe to 10 different streaming services to watch what I want (which is often just background noise while I work), which is what the industry is moving to.