silly question this is but whats the whole point for SQI arena anyway? let me rephrase, what do you get besides MVP drops? does it unlock a quest or item? 
It gives you access to Valhalla where you can make your class' Special Quest Item(s).
As long as you complete the requirements, and you can only make one that is corresponding to your class.
For example, if you complete it using a Gypsy, you will be able to make Belmont whip once.
You can also make a number of accessories which don't depend on your class, like kahos, brys, meg, and sleips[0] and/or [1].