Why is Borax used as a primary standard?
Yes borax is a hydrate, but the reason it is favored for titration is that it gives consistent results, which to most scientists, is far more important than accuracy.  

The $\ce{ .\!10H2O}$ hydration number will remain constant and will not fluctuate to $\mathrm{11}$ or $\mathrm{9}$ within normal temperature and humidity ranges. This means that per mass the same amount of sodium ions are dispensed every time and gives the same results time after time and experiment after experiment even with different batches of powder. Also the tetraborate conjugate base is very weak, which is ideal for a titration agent as well.