In a perfect salad, the greens are lightly and evenly coated with dressing. This is best accomplished by tossing the salad in a large bowl using tongs.
When the salad is served with dressing in a little cup on the side, you are forced to try to mix the dressing in at the table. The serving bowl is way to small for this task, and your dinner utensils aren't the right tool for the job.
No one would serve pasta with the sauce on the side. Why do they think it's ok to insist on serving salads with dressing on the side?