Benefits of vinaigrette vs. pouring oil and vinegar separately on salad?
A vinaigrette is not a stable emulsion so it will eventually separate- however it will stay together long enough for the salad to be immediately served and eaten.
I find that pouring the oil and acid separately creates a salad with a mouthful of olive oil coating the leaves and pool of vinegar at the bottom of the plate.

It's true that the oil in a vinaigrette will eventually wilt the salad but that is why it should be added at the last minute or even after the salads have been portioned.