Well, this is getting a bit frustrating. I tried to replicate the phenomenon, but after mucking around, nothing I import using "Import Playlist" has any value in "Number Play Counts". So I decided to uninstall and completely delete the J River folder a reinstall, since I had been importing files and then File->Library->Clear Library many times.
Having done that, then I imported the playlist shown below (I will show the first entry to keep it compact). As you can see from the XML, the file hasn't been played in quite sometime. The modification data may around the time I upgraded iTunes to the build I'm using now. After importing this XML file into Media Center, I see that the Ratings and Last Played fields are correct, but the Number Plays field is blank.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "
http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Major Version</key><integer>1</integer>
<key>Minor Version</key><integer>1</integer>
<key>Application Version</key><string>7.6</string>
<key>Features</key><integer>5</integer>
<key>Show Content Ratings</key><true/>
<key>Music Folder</key><string>file://localhost/K:/James'%20Documents/My%20Music/iTunes/iTunes%20Music/</string>
<key>Library Persistent ID</key><string>3F67715071D7DE3E</string>
<key>Tracks</key>
<dict>
<key>1593</key>
<dict>
<key>Track ID</key><integer>1593</integer>
<key>Name</key><string>Lies Of Serpents, A River Of Tears</string>
<key>Artist</key><string>Zao</string>
<key>Album</key><string>Where Blood And Fire Bring Rest</string>
<key>Genre</key><string>Metalcore</string>
<key>Kind</key><string>MPEG audio file</string>
<key>Size</key><integer>2553735</integer>
<key>Total Time</key><integer>159425</integer>
<key>Disc Number</key><integer>1</integer>
<key>Disc Count</key><integer>1</integer>
<key>Track Number</key><integer>1</integer>
<key>Track Count</key><integer>10</integer>
<key>Year</key><integer>1998</integer>
<key>Date Modified</key><date>2008-01-25T00:40:43Z</date>
<key>Date Added</key><date>2003-11-03T07:30:49Z</date>
<key>Bit Rate</key><integer>128</integer>
<key>Sample Rate</key><integer>44100</integer>
<key>Equalizer</key><string>Rock</string>
<key>Comments</key><string> <Complete Album> <Christian> <Metal> <Metalcore></string>
<key>Play Count</key><integer>13</integer>
<key>Play Date</key><integer>3266759248</integer>
<key>Play Date UTC</key><date>2007-07-09T00:07:28Z</date>
<key>Rating</key><integer>100</integer>
<key>Album Rating</key><integer>100</integer>
<key>Album Rating Computed</key><true/>
<key>Normalization</key><integer>16166</integer>
<key>Persistent ID</key><string>EDD33643E0188850</string>
<key>Track Type</key><string>File</string>
<key>Location</key><string>file://localhost/K:/James'%20Documents/My%20Music/iTunes/iTunes%20Music/Zao/Where%20Blood%20And%20Fire%20Bring%20Rest/01%20Lies%20Of%20Serpents,%20A%20River%20Of%20Tear.mp3</string>
<key>File Folder Count</key><integer>4</integer>
<key>Library Folder Count</key><integer>1</integer>