The right dressing, and the right amount of it is a very personal thing too, which is another strike against pre-dressing of salads. I love a good balsamic dressing, but it wouldn't go well with some things, where an italian or french dressing would work much better, since balsamic is a much stronger flavour and good at overpowering other things.

The only issue with adding the dressing yourself is the mixing it through the salad to get every bit of the salad to know the dressing. Not so bad if the salad is contained in it's own little bowl, but difficult on a plate with the other bits of the meal