Pretty much this.
Personally if a healer is able to dps along with healing they should. Just standing there is not taking your class to its full potential. Being kicked because of it, is just dumb though.. If im doing a speed run and I see the whm with full mp, I also expect you to throw some dps. Theres no reason not to. Its helping your group out. Your a support class- with healing, buffs and dmg spells.