I don't remember any "required" quests that require a mage(and not have a required party member that is one), but they are handy to have for the energize points which are required for a few sidequests. They are also a nice punch of power if built well. I do usually like to have a rogue, mage and warrior in the party at all times when I play, but since I play mostly mages it is easy for me. I also find it fun to mix and match different characters as each have different dialogs when paired up.....
You will either really like Dorian, or hate him....personally I find him a really good character and a good mage character, I don't usually bring him along except for my occasional rogue or warrior play as I don't usually run two mages in a party.
Edited for clarification.
Community ManagerSpeaking only from memory on this one, and someone may be more rememberable on specific quests, but there are only a handful of them that would even "require" a mage to be in the party past the ones you listed. The main reasons are choices and dialogue.
For example (Without Spoilers Given) - In Hushed Whispers - This one has some choices, and based on who is in your party, dialogue, friendships and all that may change. I love story games and definitely want to experience it my own way, but speaking from experience, on some missions you have to be careful on depending on your own playstyle for story purposes.
