I must be missing something. I can lie in mine in the rain and be dry. What happens if it's windy and raining with tarp? Rain comes around the sides, no?
if it's windy and raining with a tarp, you either:
pitch it low, aka "storm pitch"
pitch it side on to the wind and rain, going over you, aka "lean-to"
because it's not touching you and your sleeping bag
so breathability isn't an issue
so it can be 100% water proof
if you pitch it even half-decently not having sides is no disadvantage