HI Katia,
Im not an expert, but I think the problem is that they just add more sugar to the dressing, so you really arent doing yourself any favors.
I have found that if I use regular salad dressing, but keep it on the side and dip my salad in it, I use much less. Its pretty easy to cut out a couple grams of fat here and there to make up the difference, and then your not getting all that extra sugar.
Have a good day
Jayne