Vastly changeable conditions: on a day ride, yes, but not on most day rides - in the south east of England at least - let alone on shorter trips.
Cyclists are wearing waterproofs even on days with no chance of precipitation, not particularly deliberately, but because their windproof also happens to be waterproof. My point is that there are too many of these sort of garments on the market and not enough non-waterproof (ie. very breathable) ones. There are exceptions, but not enough, and I can't find one I really like any more.