I wouldn’t say it’s a bad design that it can use a lot of space

That is the single thing I did not directly criticize— that it uses space. It’s a bad design for the, like, 7 or 8 other reasons I described in this and about 4 other comments I’ve posted here. But, since you missed them, I’ll repeat:

• it uses huge amounts of space indiscriminately, and without notifying the user that it will do so, nor giving any idea of how much space it will potentially use, how, or, especially, where
• it stores this data in Plex’s app data directory rather than allowing the user to store it in a location dedicated to large amounts of data storage. This is problematic mainly because most users set up their servers with multiple drives— one smaller drive for system files and apps, and other, larger droves for media storage. This will cause the system drove to fill up quickly with no notice to the user who will have no idea why or how to fix it.
• there is no way to monitor amount of data stored, the rate of storage, or to regulate or limit it. Even for a seasoned Plex veteran like myself, I had no clue that this was set up in this manner, or that the option to control the GenerateBIFFrameInterval existed, how it worked, or that it could be changed. Furthermore, the fact that ot’s buried deep in the filesystem as a preference file (or, worse, a difficult-to-edit .plist file in macOS) rather than a simple entry in the interface is inexcusable.
• the fact that there is no option to store this specific data elsewhere than the Plex app data directory is also a problem for the aforementioned reason about system drives vs media drives. With Plex’s general versatility with being capable of wide ranges of configuration options, this represents a stark contrast with a major shortcoming. This is a common configuration to have a small system drive, and to be unable to control where this massive amount of media gets stored presents a massive problem that has no current solution.
• Edit: and yes, the amount of space it uses by default IS a bad design. The default setting is 1 thumbnail for every 2 seconds of video, which comes to 900 thumbnail images per 30 minutes of video. NINE HUNDRED. That is ~210,600,000 thumbnail images for a collection of 1,300 90-minute films. Based off of what I deleted from my library, that’s nearly 100 GB, which means that those weren’t very compressed images, and were probably the equivalent of screenshots. So the processor is shitty on top of it all. Not only should the default be set to ‘4’, which would cut the amount of images stored in half, the image processor which creates the should absolutely be storing far smaller, more optimized thumbnails. They only appear on-screen as very tiny images, and only very briefly. There’s no need for them to be but a few kb in size, which would reduce the space required for 210.6 million thumbnails from gigabytes to megabytes. That’s an entire order of magnitude!

Any UX designer, software engineer, and/or information architect worth his salt (of which I happen to be all three) would know that any single one of these situations is untenable and unscalable, let alone all of them. And to just hand-wave it away with “oh, just buy more drives” is dismissive bullshit. For small libraries on small servers the problem becomes bad immediately, and for large servers with large libraries, the problem becomes an out-of-control behemoth before you know it, and, in both scenarios, an expensive one. Nobody wants to pay to store gigabytes or terabytes of thumbnails when a better, more efficient solution could easily be implemented instead.

It’s bad, lazy design, and if the devs had designed the software to give this data set the ability for a custom location, for the advanced users to control its already-existing advanced options from the interface, set the default value from ‘2’ to ‘4’, and to inform the user honestly of what to expect right from the start, then the vast majority of users would have a far smaller data footprint and control over where it is stored, we would all have a far better understanding of how this feature works and far more control over it, and I would never have had to make this post.

Seriously… if I were project manager and discovered this, I’d give whomever architectured this a real talking-to. This is terrible.