I have used foam blocks directly on roof of a car a few times and my experience is similar to what Maureen described. You can probably get away with them in a pinch for short trips in dry weather at low speeds but problems are significant under other conditions.
In wet weather, water will wick down the straps into the car even if the door seals well.
At highway speeds or even long trips at low speeds, I found that the foam blocks slide on the roof even with bow and stern lines. If the straps are made tight, sliding is reduced but the roof will be dented more (not permanently for me). At highway speeds, the kayak is likely to shift diagonally.