I remember it not being great in PVP on live at least at the time of the 2001 Druid BotB. It was essentially a wasted 9 seconds against people wearing MR gear, but you are correct that it shouldn't be resisted 100% of the time.
The spell file indicates some changes to PvE and PVP resist adjusts and caps in 2002 and 2004. Maybe that is where the resist mod comes in?
http://lucy.allakhazam.com/spellhist...26&source=Live
2004-07-14 09:45 Changed Pvpresistcap from 100 to -100
2004-06-02 11:16 Changed Unknown182 from to 1
2004-06-02 11:16 Changed Unknown183 from to 30
2004-05-12 10:26 Changed Pvpresistcap from 0 to 100
2004-05-12 10:26 Changed Pvpresistcalc from 0 to 100
2004-05-05 11:15 Changed Pvpresistbase from 0 to -100
2002-09-26 11:10 Changed Resistadj from 0 to -100