I spend several dozen hours riding in the rain/wet every year. More than the vast majority of cyclists, I'm sure.

My own personal experience is that some tread is much better than no tread when riding on wet pavement. Most recently I've been riding 650b Grand Bois Hetres. These tires have longitudinal ribbing that can be seen if you zoom in on this picture: https://drandalls.files.wordpress.co...2/img_1174.jpg

When the ribs wear down the tire becomes completely smooth. Compared to when the ribs were visible and tactile I've noticed no change in behavior when riding in dry weather but the change in wet weather is very noticeable. The rear end has a tendency to break loose much earlier than before, I also get slight rear slippage when climbing out of the saddle that did not occur previously.

I noticed the same thing when riding 700c tires. Schwalbe's Lugano tire has a light file tread that worked very well with wet and dry pavement, when it wore down to slick the performance in the wet dropped off significantly.

I generally take it easy in the rain so the difference in traction just makes me more cautious. If I was planning on any performance type riding I would definitely make sure my tires had visible tread. Even if very thin.