I have the same problem of constant library updating. It makes WMP12 almost useless, considering that my music is on an external USB drive (USB is CPU intensive, and the drive is louder than an internal drive.)
this is certainly undesired behavior, having no option to tell wmp how to handle the library. It should have just a simple monitor to update after file additions, and file changes. nothing more.
Maybe Microsoft intended users to never close WMP, and never shut the computer down...