I'm not sure I understand this question. Are you saying that you never feel the tires slip when you're riding? Obviously that's something to be avoided, but it happens if you ride in enough diverse conditions and I think most commuters do at least once in a while. (Incidentally, one of the reasons I recommend cyclocross racing for even people who aren't interested in racing is that you get to spend an awful lot of time feeling your tires slide, and so when it happens in normal riding you're prepared and can handle small slips.)

Usually when my tires slip it's in conditions that no stud-free tire would prevent -- wet leaves, loose gravel, mud on the pavement, black ice, etc. The grip comes into play when you get back onto a solid surface. You can feel a difference in how quickly the tire reengages. I don't think any tire on the market would slip on dry, smooth pavement, but add some water and it starts to become a possibility. Depending on how a tire responds in the unpreventable slipping conditions, I may be less cautious when riding on roads that are just wet.