Welcome, Groovey.
Thomas pretty well sums it up. Except for mounting a second arm with a mono cart, there is not much to justify a second arm.
But many of us are afflicted with a need for more performance. So, in our twisted minds, that is enought to justify adding more arms. For some, it's just a need to experiment, for some it is a need to optimize for one type of music vs. another. For some it's just a need for more toys! :) I'll probably be adding a second arm in the next year.
If you are concidering a Teres or any other table with multi-arm capability, you owe it to yourself to audition Galibier as well. I do own one and highly recommend them.
Best,
Dan