I use full-coverage tarps almost exclusively and always take a bivvy. You'll find that without snow (or dead leaves, etc.) to bank around edges of such a tarp, wind chill can occur, even with edges carefully staked to ground. To say nothing of rain splash, drips, or unexpected drainage patterns.

These alone are reasons enough to include bivvy with these shelters. Also with floorless shelter, bivvy makes it possible to dispense with ground sheet, a potentially significant weight savings. This may or may not suit the individual's taste.

Also, the added warmth and weight of a bivvy can, to some extent, be subtracted from the weight and warmth required of a sleeping bag.

Also, bivvy offers peace of mind for temporary bag storage in camp. Shelter can blow down, camp invaded by plague of centipedes and arachnids and maggots, and bag remains clean and dry inside bivvy, which also makes stuff-sack redundant and useless.

With snow, a full-coverage tarp can be fully sealed from wind and is an entirely effective winter shelter.

At the other extreme, if you want a set-up where you can look around outside, just raise the edges a few feet off the ground and leave the door open.