I had the same issue with the sidecars, as my husband prefers to browse using Windows rather than Media Center. I however, browse with MC and want everything tagged and looking sharp. So I investigated ways to get rid of the sidecars to keep him happy and to keep my MC pretty at the same time. Now I'm sure this is not the official answer to the problem, but I found that as long as the sidecar files remained
anywhere on my computer, the display info in MC was unchanged. So I just moved 'em all to the Cover Art folder, and BAM, we were both happy. This solution applied to network share files as well. There are some considerations. though, if you make any extensive changes to your library.
I learned, quite by accident, that if you want to move files to a new directory, do it using "Rename, Move & Copy Files" under "Library Tools." If you move the files in the conventional manner, all file info will be gone in MC.
Also, if you get the urge to clean house and start fresh with a new MC library, but you plan on importing the files you had in the old library, those files won't retain the sidecar info.
Happily, I learned that it was possible to transfer MC to a different computer without losing any sidecar info. I had to depart with my main comp for some warranty work
but didn't want to be without MC, so here's what I did. First, I used the "Library Back-up" function in MC on my main comp, then I installed MC and ran the "Library Restore" function in MC on my old back-up comp . (Sidenote: I also checked the box for the "Settings Back-Up (experimental)," and it worked GREAT, restored all my settings, except for my equalizer settings.) All the files were organized just the way they were supposed to be, but alas, no sidecar info. So I decided to try copying the contents of the main comp's "Cover Art" folder (which, remember, is where I had stashed the sidecar files) to the "Cover Art" folder on the other comp. Still no luck. I racked my brain for awhile, and then had a DOH! moment and figured it out. The "Image File" field in the file tags was pointing to the wrong directory, as the drive letter MC is installed on is not the same on each comp. Actually, the tags were pointing to the wrong directory for the actual files as well, since I had already imported them from a back up from the broken comp, but I used "Rename, Move & Copy," this time changing only the directory of the files to fix that problem.
The "Image File" field is not editable, so I wasn't able to redirect that way, and since I had restored my library from a different comp with different drive letters, I had to then go to "Tools-Options" and enter the new directory for Cover Art storage. Once I did that, I was able to select all affected files, and use "Cover Art-Quick Find in Cover Art Directory" and everything was restored beautifully. I know there have been a few complaints on the forum about "Quick Find" not working correctly, but it worked great for me.
I suppose these last steps would work in the "new library" scenario above, but then it wouldn't really be a new library, would it?
Hope this helps until an "official" answer comes along.