A full-on tent with floor and netting, or something similar, is wanted for really severe insects.
Mosquito problems are variable and to some extent, swatting comes with the territory.

This doesn't affect question of whether some sort of bivvy sack is desirable accoutrement for back packing, provided the additional weight is well thought out. Nor whether tarp tent is viable for many or perhaps most, camping circumstances. The netting on a bivvy does work as intended, but it's a rather minimal form of protection from insects.

Link below considers bivvys as substitute for tent, and as such, doesn't at all consider my viewpoint which I sought to explain in some detail in earlier post... But given this limitation, link offers full and highly credible discussion of who might benefit from a bivouac sack:

http://www.psychovertical.com/?bivibags

Given unnecessarily limited terms of the analysis, I agree that solo backpackers should not use them. It says:

No; the problem with a bivvy bag is that you can't do anything within one apart from sleep. Get caught out in stormy weather and you can't cook and just try getting into your dry sleeping bag in the rain without getting it wet. Hooped bivvy bags are often bought by these users, but unless they're proper mini tents (like the Outdoor Designs Racer), they're even worse than a bivvy bag, being just as useless but heavier and more costly. For this user the best bet is either a single person tent (warmer, bigger, bugproof and a joy to live in), or if weight and pack size is important then just take the fly sheet (Macpac Microlite fly for example).

In my considered view, single-person tents are hell to live in after a few days, and obviously just a fly provides limited insect protection.