A V-Twin has a better torque curve (max torque is at a lower rpm than on a parallel twin), peak HP is at a lower rpm, and it is a smoother engine. It doesn't need balance shafts. A parallel twin is also much less expensive to build, hence the reason it is used so often.Why is a parallel twin a step-down from a V-twin?
Take for example the CB500. That parallel twin engine has peak HP and torque at 9500 RPM and 8000 RPM respectively, while the NT has peaks at 8000 and 6500. I've had both bikes, and the NT is a much more enjoyable engine. The CB engine has to be wound up high to get any life out of it.