My DS is 19 months and we do use toddler formula.
My situation is that we've breastfed from birth, but I've always had genuine low supply (long story...but basically my supply did not increase despite trying a whole range of things and so my DS needed formula supplement in order to gain weight).
At the 12 month mark I did offer him other types of milk (cow, almond, rice, coconut) but he did drink it, so we switched instead to a toddler formula.
We still continue to nurse on demand, but I notice that without giving him the toddler formula, his diaper output decreases rapidly. My DS does not (ie by his own choice/will) drink any other liquids besides breastmilk and toddler formula, so it is important that he gets enough fluids to stay hydrated. He is also a very picky eater and eats only a small amount of a very limited range of food. So in my DS' case, toddler formula is just one of several foods that he takes in, this is the way I look at it.
I would say, to use your own judgement based on your knowledge of your DD's food and fluid intake to determine if your she needs fluids other than the breastmilk she gets. Remember, they do need to be hydrated as well as fed.
I agree that if your child is eating well and taking in sufficient fluids, it is probably unnecessary.
I would disagree that toddler formula is "gimmicky" - there is a genuine time and place for it - such as the situation I described above.