From Rashere, Lead Designer of EverQuest on April 13, 2006:
"There's a rationale for non-trackable mobs? Personally, I abhor them and fix them whenever I find them. The only things that should be non-trackable, imo, are NPCs you couldn't interact with as a player just so they aren't cluttering up the tracking list. There may be an exception here or there under certain situations, but they should be very, very rare.
I'm generating a list of all NPCs flagged as nontrackable right now. Most of them likely shouldn't be, so will be changed when I'm done."
It somehow got mixed into a thread about the Endless Quiver,
Update: April 19, 2006 -
This didn't make the patch notes, but the majority of NPCs that weren't trackable before should be trackable now.