I found something interesting things about this. This is from a guilds DKP page:
http://www.silentredemption.org/mobinfo.cgi?mid=17
http://www.silentredemption.org/mobinfo.cgi?mid=15
http://www.silentredemption.org/mobinfo.cgi?mid=22
That at least shows them not dropping all the time. This is from 1 guild so percentages should be taken with a grain of salt.