they are fine - i use mine to get the majority of the water off, then use a chamois for the rest of the car.
The only way you'd cause any scratches with one is if you hadn't washed and rinsed the car well enough - there is no other reason, grit is a lot less likely to stick to silcone than a chamois!
best to go over the missed bits with a chamois just to give it a nicer finish, you might leave small streaks if you only use the blade.
Nick
edit - and to answer the main question, I use mine everytime i wash my car, which doesn't even have any paint swirls on it, let alone scratches that the water blades "cause"