1. Melee units are useful, because they will utterly destroy anything on the ground that isn't strong against them.
2. While I agree that mages may have a slight edge, particularly archmages, the other units are still useful, particularly alchemists and protectorate troops. the only useless units in my opinion, are engineers/technomancers and most guild units - minimal damaging capability :(
In my opinion the best starter character is the necromancer - healing, quite good ranged attack and a summon that lets you kill many enemies without getting close to them