I use a sil poncho in warmer weather - I use it instead of a rain jacket just so I don't sweat so much. I don't have a problem with it wetting out/through - I stay dry with it.
I DON'T also use the poncho as a shelter for reasons already brought up - like having to bring an additional jacket and/or tarp, and rain geting in with so small a tarp over your stuff.

So - I bring a bigger tarp to put the bivy under, and use either the sil poncho in warmer weather, or a rain jacket for cooler/colder weather to ewar around camp when needed.