I agree with both LiberalSquirrel and archvile93, it's amatter of cultural stereotypes and gameplay balance. If mages could act as every other member of the party at the same time it would be pointless to play as any other class. One of the best examples is Merrill in DA2, she is a good support char and can resist a serious ammount of damage. Luckily the only thing she can't do is healing, otherwise she would monopolize the whole game. The only good example of battlemages I've seen was Wynne in DA:O, she would use some of her magic to be able to fight in melee form while still acting as a formidable healer, I think she was bit overpowered though.
I recommend you all to check on Titan Quest and its expansion (it fixes maaany bugs and adds a lot of great content), the class system allows the player to mix a couple of classes to get a different char with every possible combination, it lets you play as melee, ranged physical, ranged magical, mixed magical and melee or magical and ranged, use lots of pets or go absolutely solo. That is without counting all the mods the community has made as the game is fully moddable since its launch. Those, ammong other stuff, add several classes more.