Yes. As you will see above I agreed both items serve different purposes, just as you said. And yes, I have been in the rain with and without a tarp. without a tarp you have no place to sort through your pack,get undressed/dressed,or cook or anything when it is raining. I did find enough space inside the bivy to read and that surprised me.

Perhaps another subject, but related: My best (Gortex) bivy is a bit heavy at 24 ounces (682g) and my lightest sil-nylon tent is only 8 ounces (227g) more than that so given the convenience of a tent and the requirement of a small tarp over the bivy I never packpack with the Gortex Bivy any longer. Thanks for your thoughts.
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Jim M