On the other hand, using sidecar files would essentially mean giving up freedom to move/rename your files. Sure, you could move/rename the metadata files manually, but it'd be much more work, and you're bound to mess up sooner or later.
I'm not sure how to solve both those points without making the application "own" the files (as e.g. Camlistore does).
Think of things like any of the cloud drives, bittorrent, screen-sharing or IM software. Are they aware of ADS (or equivalent for that platform). What would the right thing to do be when syncing between platforms that have support, and ones that do not?
However file metadata is usually pretty poorly understood by most developers (based on a personal sample of my past experiences) and file system resource forks are even less understood...
Perfect is the enemy of good and all that sort of thing ...
Outside of Office sometimes trying to encourage using it to make searching easier and Windows sometimes using it for ID3 tags from audio files, I've never really seen anyone try to use it.
On GNU/Linux systems maybe, it isn't part of POSIX.
How will you enforce something like that to all UNIX eco-system?
Otherwise you get inconsistent behavior such as metadata being synced when you copy entire folders, but not when you copy individual files. Then you have to rely on even more hacks to get around that, like detecting when a file is copied and automatically copying the corresponding dotfile. Sooner or later you end up with a complete mess.
1) This is not default behaviour, you have to specifically enable this
2) They actually note that if you copy files outside their app, metadata does not get copied, it's doubtful they'll fix it
So basically, NOFIX, working as intended. Not poor design.
* Ability to save tags in sidecar files
Here is their feature page: https://www.tagspaces.org/products/pro/#tagSidecarSaving