I thought it didn't affect me the way I'm working but it does.
I see from another topic you have some issues with cover art on upgrading from MC16 to MC17. I'm not sure what that's about and won't comment here, but I'm sure things will work fine for you once those issues are resolved.
Let's keep this in perspective. We're talking about where the program saves data files—on the system's primary data storage device. We normally don't care much about such things, as long as they're rational and consistent, and allow things like backup routines to work properly. We can, of course, have our own independent interest in the same files, and therefore in how they are saved. But it doesn't follow that entitles us to claim the method used by the program is inferior, wrong or an unacceptable inconvenience. Saving meta data files beside media files is simple, rational and reliable. If there were no history here, it would be inconceivable this would not be the first choice of how it should be done, or that there could be any compelling reason for an optional alternative. The current controversy is good evidence of why such unnecessary options should never be allowed.
There is an issue with using removable HDD for media storage, but that's about recognizing handling them as removable storage. Ignoring that issue and insisting on some workaround for it's impact on cover art doesn't make sense.
I would be fine with the new solution if I could also save my cover art in to a back-up folder too.
If you're using a conventional backup system to backup media, the cover art will automatically be backup according to how you've configured that routine (daily, weekly, multiple versions, etc.). With the meta data saved with and beside the media, there can be no errors or confusion about what is being backed up or how to restore it. It you don't have such a backup system and want to backup only the cover art, you can use a simple batch file or one of a wide variety of free copy, synchronization or backup utilities to backup the image files in a video media folder structure. Such a routine could save multiple versions of each image (which is essential for recovering from doing something stupid), and be scheduled to run automatically as often as you consider necessary. That has to be a much better form of backup than whatever you have now.
I can see the point of having all the files into the same folder (cover art, sidecar.xml) but I have to think really good about everything before making decissions I can't turn back easily.
I suggest you focus on getting your cover art restored in MC17. Once you have, the current system for managing cover art will work well for you.