huh.
I'm merging a wolfmame151.xml, a (probably stale) controls.xml, catver and nplayer from version 150 while I write this. I just opened the output file in notepad++ and I see tags for nplayers, veradded, category, etc in the data.
here's the first entry:
<mame build="0.151[W] (Nov 6 2013)" debug="no" mameconfig="10">
<game name="005" sourcefile="segag80r.c" sampleof="005">
<description>005</description>
<year>1981</year>
<manufacturer>Sega</manufacturer>
<category>Maze / Shooter Small</category>
<nplayers>2P alt</nplayers>
<verAdded>.030</verAdded>
It may be best if you zip up your input files, including your mame.xml file and send them to me (email address is on the romlister website) or post them here or wherever you can so I can get them and try and duplicate what you're seeing.