I had the same issue where if your advertisement was set to "Access Content Directly from a Distribution point..." the XCOPY step would copy ALL Contents of my $SMSPKGx share to the USMTBITS folder (NOT what was desired). I DESIRE to keep the advertisment to this option (Access Content Directly from Distribution point) and I found this "bug" work around (Link: http://blogs.technet.com/osd/archive/2009/12/31/why-do-all-packages-get-cached-when-steps-to-cache-content-from-a-specific-package-run.aspx)
Bottom line to correct is to change your XCOPY to use: xcopy %_SMSTSabc12345%* %USMTDrive%\USMTbits /herciy (of course changing the abc12345 to the Package ID of your USMT package). This will ONLY copy the USMT package contents to your local USMTBITS folder and NOT the entire contents of your SMSPKG share.
