There are pros and cons for having LR write metadata into .XMP files. It takes up disk space and slows things down. However, if the catalog is corrupted and there is no backup, that metadata would prevent complete loss of one's editing work. Personally, I do have LR save the metadata to .XMP because I still use Bridge a great deal with Photoshop.
Bill
Just did an experiment by dragging and dropping a copy of a Raw file with saved LR 4.4 xmp edits from LR's Library Grid thumbnail layout into a Bridge CS5 image folder (not written to LR's Catalogue) and the Raw file retains the xmp icon and its LR preview in Bridge's Filmstrip thumbnail and Main preview. I realize if I open this LR edited image in ACR 6.7 I'll lose the PV2012 settings with changes of some degree to the preview.
Just wanted to see if stuff gets corrupted, freezes, crashes or I get any dialog box warnings from either Bridge or LR. So far nothing.
The only way this would work in reverse with a drag/drop from Bridge CS5 Picture folder directory into LR sidebar Library folder directory (which I've limited to a few folders to keep LR's CAT from expanding its directory to include all of my entire Mac OS Picture folder) is to invoke "Synchronize Folder..." in LR. Otherwise LR won't show it since it uses its catalogue to see folders and folder content.
Please correct me if I'm wrong and if it's NOT best practices to do this kind of moving stuff with LR.