Increasing magicka has a somewhat exponential benefit to it. Because Magicka regenerates as a percentage of your max, and not a static value, increasing magicka improves your magicka recovery, which means both in-and-out of battle, you have significantly greater spellcasting endurance if you fortify it. At 200 magicka, you've got double the base amount, and double the regeneration speed of a person at 100.
If your primary means of resolving conflicts is spellcasting, you should aim for a value much higher than 200. If you're just using your magicka to skill up, you can actually stay at 100 but be prepared to invest heavily on special "Skill up" sets that optimize your magicka, regeneration speed and casting cost.