Because interest accrues on your principal balance, capitalization will cause more interest to accrue over time than if you had paid the interest. It will also increase your monthly payment under most repayment plans.
That's not that great, but I understand it. What if I'm pursuing loan forgiveness, though?
Then any period of deferment or forbearance likely will not count toward your forgiveness requirements. This means you'll stop making progress toward forgiveness until you resume repayment.