I am gonna go ahead and reiterate that I absolutely love alteration. I would go ahead and say it is my favorite skill and spell set in the game. For me, you cannot beat passive perks such as Magic Resistance and Atronach (30% spell absorption). I try to work in this skill in almost all of my builds, something I may try to stop, but I just can't do it.
For a mage wearing robes, it is invaluable to have an armor source that can reach up to 300 or more depending on your standing stone or wards. In terms of effectiveness, it is widely considered not as good as normal armor in protection value, but with high levels and stronger spells, it can be useful against most enemies.