aside from bug protection, a bivy is unnecessary. if you have a spacious tarp (6x9 at least), you don't necessarily need to only sew to the liner, but it could help some. Having some permanent way to stop drafts like buckles or lines that tighten the quilt will help with drafts, but i haven't found a need for them yet. it would be best just to test out your preferences in inclimate weather because everyone is different, as are peoples' definitions of 'necessary'.
I still haven't figured out why people are so adamant about bivies with quilts and small tarps. I think if you are having draft problems, you should convert your quilt to a topbag (lighter, more breathable and just as protective if you use the aforementioned construction) and if you are worried about splash, you should bring adequate shelter in the first place.