Yes, Avengers are better than straight Druids, and for soloing they have their advantages, but the mix of their bonus spells doesn't quite make sense.
IMO, Avengers are overrated because they really only make sense in one setting, and that is as the only caster in a party. Avengers are basically a light version of a Mage/Cleric, much like a Stalker is Fighter/Thief light, and a Blade is a Fighter/Mage light, etc.
But the problem is that if you make the Avenger the only caster in your party, then you miss out on key spells that you would want in a Fighter heavy party. Those key spells are Prot from Petrification and Haste.
So, IMO, the Avenger's level 1 spell should be Prot from Petrification and their level 3 spell should be Haste. With those two spells a party like an Avenger with 3 to 5 other Fighter and/or Thief classes would make some sense, but without Haste such a party just isn't justifiable.
And, IMO, when you put an Avenger in a party with other Arcane casters, their value goes way down because their kit doesn't really bring anything new to the group. In a group with Arcane casters I think Totemic Druids make far more sense. Otherwise their main advantage is just providing more access to Improved Invisibility.