At least in the West, once the sun goes down and cools off, the bugs are not much of an issue. So there is really little need for a bug net just for sleeping.

And a the extra coverage of a larger tarp should be less than the weight of a bivy.

So, for some conditions, it is the lowest weight option to go with larger tarp. Although the lightest option is a small tarp with no bivy, and just hope for no blowing rain storms. For a cuben tarp, we are only talking about a couple ounce difference, though.