Okay, since my Jeep was pictured, I will jump in here.
There are a number of reasons to upgrade to Long arms..
1 - The ride will be greatly improved due to the control arms being closer to the stock angles. (on a stock Jeep, the control arms are very close to parallel with the frame/unibody. When you lift it, they start to angle downwards, thus making the ride more harsh, causes more stress on the control arm bushings and, eventually wearing them out which I've found out from personal experience, will cause death wobble.
2 - More flex, and less stress on the frame/unibody
3 - It is easier to dial in you suspension with long arms and prevent death wobble.
4 - "Cause all the cool kids are doing it".