if Hawke is not a mage there is also no need in Anders. because warrior Hawke has better cc and damage output. Don't know about rogue Hawke though.
and there is no companion that much more powerful than the others, all of them are gimps.
mvp - is Hawke.
First of all, all mages have higher damage output than warriors, that's simply a gameplay factor.
Single target damage: Rogues > Mages > warriors
AOE damage: Mages > Warriors > Rogues
Toughness: Warriors > Rogues > Mages
There is no room for argument here, this is just how the game works. Mages can bring support to the table. Merril can't really do it because she is offensive mage. Its up to mage Hawke or Anders to do it and if Hawke is not a mage then Anders is the only one and he is quite good at it. As I said apart from healings he can increse the party's damage output for 450% permanently.
Vengeance built Anders has highest possible mage damage output in the game if you get him the right talents and stats but he is very squishy in that form, dealings more damage at expense of defense. People tend to ignore their companions for the most time and focus on their protagonist, which is understandable but claiming that's fact is bogus. Aveline will always be better tank than Hawke, Varric will be a better Archer and Anders will deal more damage. if you notice none of Mage Hawke's extra spec have any real damage ability. Hawke will still have better utility and crowd control but Anders has cooldown reduction which is game breaking in terms of damage.
Also Aveline is better than fenris, Varric is better than Sebastian, Anders is better than Merril. So yes some companions can be better than others.