I played a hunter solo for about 2.5 years straight - the pet "growl" during that time was always considered "broken" by the hunter community because of how strong it was.

AFAIK - this is how "taunt" works.

Tank has 100 threat points.
Mage passes tank with 101 threat (101 being the highest number on the table)
Tank Taunts which does "101 + 1" = Tank now has aggro. (this is why DPS should back off for a second as tank reaquires target.

Now pet taunt (AFAIK) works the same way. So tank now has 102, pet taunts now has 103. True tanks can easily out threat pet aggro - but if the pet growls - its "tank aggro" +1 all over again.

Perhaps this has changed over the years - I really don't know. Maybe growl isn't on the DR table as taunt is? (which would seem silly - you'd see end game guilds ping ponging bosses around with pets)