I used a rain poncho extensively for touring but that was almost 40 years ago when I rode from Finland to Spain on a 5 month tour. It worked great in all sorts of weather and wind. There was no, ahem, gore tex back then so you used coated nylon. At least with a bike poncho you had good air circulation and could sweat properly. I had wool tights which kept my legs warm. Oh yeah I saw a lot of rain. It rained right around 30 days straight when touring through Sweden (the locals told me it was one of the worst summers on record).

Exped makes 2 rain ponchos (one is ultralight),

Daypack & Bike Poncho | Exped USA

I have no experience with this but their mattresses are first class.

J and G makes a rain cape which is very reasonably priced,

Rain Capes from People Who Really Know Rain Capes!

I agree with post 3 though that one advantage of a rain jacket is that it is a fine outer layer when the temp drops.