Ok, let me be a little more precise in saying why I don’t like the specs: it requires dedicated tools with insight into the XMP files for all the basic things like copying files around, backups, restoring from backup, sending edits to someone else and probably many more.
With a simple 1:1 mapping of media file to sidecar you can use the most stupid tools your operating system offers, be it cp, mv, rm or rsync for backups. All that breaks down horribly as soon as you put more than one media file’s XMP data into the same sidecar.
Since you need to put something into the individual XMP segments to identify what media file it belongs to (filename, checksum, …) you may never change that detail of your media file without editing the XMP, too, which again requires dedicated tools. So you can either not rename your image files, not change the content (for raw files that is ill advised anyway, but sidecars are used for other formats as well).
Regarding interoperability, I don’t see a problem with that. While XMP is a standard that can be understood by many programs hardly anything is interoperable in reality. Basic things like ratings are, tags are to some extent (there are application specific namespaces used already, for example by Adobe) and color labels are totally crazy. Once it comes to processing information there is basically no way to share the data between applications.
So what is left are some basic things that might be shared between applications, however as that is such a limited set almost no one does that in the free software world. And the few that actually do can use symlinks as described in other posts to get that working. And when trying to move the sidecars written by one application over to another without the intent of going back and forth (say, a camera writes a sidecar with ratings for every image it takes and I want to get that over into darktable, or I want to stop using darktable and move on to Lightroom) then a single rename step is enough.
TL;DR: I am all for following specs, even if it makes life a little harder, as interoperability is a good thing. But not for a price like this. Especially as basically no user missed that so far.
PS: I would be interested in seeing your calculations that said free graphics software had a market share of 1%. That sounds way too high. I wouldn’t even believe 1‰.