Am I overlooking something? You're supposed to repay as quickly as possible, right?
Yes and no. Repaying as quickly as possible ensures that the loan is paid off fast. On the other hand, life is about more than just the mortgage loan, be it hobbies, vacations, or other investments. So it's always a compromise.
Basically, you are financially better off with a lower repayment rate if alternative investments generate more returns after taxes (rule of thumb: return -25%) than the loan interest costs. On the other hand, you don't know what interest rates you will get in 5 years. So the calculation has some uncertainties. And larger sums in the account lead many people to imprudence and waste of money. Therefore, I would always try to find a balance.