The trouble with most pets in general is that they are more useful for the "use this pet, get this benefit" effect than they are as a tank or source of damage. The trouble with dogs/gargantuan (gargantuan especially) is that their primary benefit IS as the source of damage, so their usefulness fell off quickly since their damage output is sort of like that of followers in that they plateau at a certain point and don't really get the boost from sets (except Helltooth) that other skills get.The Dog and Gargunantuan buffs are quite needed, as they are iconic skills that sort of become worthless after a point.
--Patrick