Yea I'm not sure why holy avenger gets bashed It is very similar to pet ability with the beast master. +5 life and a +2 Melee and +1 piercing is nothing to sneeze at and a really good ability.
Pet gets you +3 life, +1 armour, +1 melee and a further +1 melee really easily.
Holy Avenger gets you +5 Life, and it sometimes gets you +2 Melee and +1 Piercing, but requires your opponent to cooperate or you to jump through hoops, and in either case often play badly to get the bonus.
Both abilities cost the same. +3 Life +1 armour is a little better than +5 life (note how Bulls Endurance is unusable outside familiars, yet Rhino Hide is used all over the place) but lets be generous and call that even. The Pet gets +2 melee for following your optimum strategy of killing the mage, whilst the Avenger is only slightly better at +2 melee and +1 Piercing, but only when they hitting something that hit you first, and if you want to hit it back. It's trivial to deny it from the mage with initiative tricks, activation orders (and some mages just won't trigger it anyway), and attacking creatures is frequently the wrong play. The Avenger bonus is commiseration prize for when you need to use it, not something you want to use - you want to beat down the mage as fast as possible!
The two abilities are not even close. If Avenger was free, then Pet would still be better, as it is, Avenger is waaay worse.