Unless, of course, the boss or trash mobs do some nasty AE abilities on the melee which would require you to move the mobs out and you can't because the pet is taunting every 8 seconds.
Also, for those of you that don't know - before MoP the taunt wasn't that big of a deal - it was a numerical threat value that was rather high that any tank could easily outdps. So, no biggie there. With MoP, the taunt on ALL pets is equal to a tank taunt. THAT is what makes it so freakin' irritating.
But yeah, now, I just let the pet tank - allows me a little time to grab my drink and just white damage for awhile
It's the hunter or warlock that's losing dps by wasting GCD cycles throwing heals at it to keep it up.
EDIT: And I will never understand why hunters don't MD every time it's up - especially on trash packs. You're not losing anything. Why not help out? Get in the habit to start off with and it'll serve you very well later down the line when it's 2nd nature to ya.