As my post above indicates, this effect is really important. The plaque biofilm is really sticky. I'm not sure what's going on in GP's case, but when using a plaque staining dye to see what's going on, it's clear that toothpaste makes a huge difference in the ability to mechanically disrupt plaque. It's amazingly hard to do without toothpaste.
If everything goes as planned:
The abrasive is more likely to scratch what's on the teeth; and
the teeth are more likely to scratch the toothpaste if everything goes as planned.