Because rank points are used for
...
...
...
ranking.
Rather than a reward for dueling it serves as proof of that.
It has been explained thoroughly when the system changed. Please don't make me again have to spend time writing a huge post after midnight (sorry but may it be one person or the other, for 3 days straight now I had to stay up writing responses) and find those... here are some places you can check.
http://www.worldduelingacademy.com/t3199-introduction-to-the-new-system
http://www.worldduelingacademy.com/t3282-how-re-the-new-changes
P.S. You can't spend things because there is no reason or properly justifiable way to spend, and I have explained that much as well.
