I wrested with this question when looking at bug protection for my Gatewood Cape.
A bivy isn't needed, but bug protection is welcome in season. The Serenity net tent for the Gatewood is 8oz and $120— nearly as much as the Gatewood at 11oz and $135.
I looked at light bivies and got a Titanium Goat Ptarmigan with the head end insect screen at 4.8oz and retailing for $100 (I got min used). I can use the bivy for cowboy camping and with my poncho as well as the Gatewood. It is lighter and far easier to deploy, simply needing a cord to keep the mesh off my face if desired. I don't do much hot weather camping and the bivy does give an extra layer of protection for the cool wet camping that is more typical for me. It allows a slightly lighter summer bag or extends the warmth of my shoulder season bag.
It doesn't have the room of a net inner and could be less comfortable in hot weather.
My $0.02