Is there any benefit to maxing out a branch of magic/abilities?

There are achievements for maxing out a school of magic. There is no in-game benefit to it,only the bragging rights. I would not focus on one school but choose whatever spells are most useful. The versatility you gain by that is one of the biggest advantages of the mage.

For warriors and rogues it makes more sense to specialize in a school as they are often mutally exclusive. If you are using a shield your two-hander skills are useless.

While there is no added benefit, typically the higher up skills/spells are more powerful, so you would want to get those as soon as possible, but no reason why you should stick solely on creation, for instance.