When I lived in Seattle, the rule for most group rides was "steady rain cancels". Light rain or intermittent rain was acceptable. In a steady rain, if the ride leaders purposefully slow down the group, it helps to make everything safer for those riders that perhaps aren't as skilled in the wet, thus making it safer for everyone.
On the other hand, the race team I was on up there rode year round (well, most people took a break in October but not due to weather), rain or shine. Only black ice on the roads would cancel rides. It all depends on the experience level of the group.