good for the desert or emergencies, tarp + bugbivy better otherwise
A waterproof/breathable bivy is a good idea for deserts, where there is the possibility of strong winds. The alternative is a freestanding tent with very strong poles to cope with the wind. I wouldn't travel where wind is common, but I will travel where wind is occasional but can be ferocious when it happens, like the southwest deserts of the United States. Stakes will not hold in desert sand and lightweight freestanding tents will break.
A bivy can also be useful in certain winter applications (snow caves), though I don't have much experience here. And of course it is useful as an emergency shelter.
Anywhere you can get stakes to hold and are sheltered from strong winds by trees, a tarp plus bugbivy is the better choice. Here is my homemade tarp + bugbivy (735g or 26oz including stakes and rear pole): http://frankrevelo.com/hiking/sewing_tarp_frontview.jpg
There are lots of commercial versions like mine. Tarp-tents are another possibility for about the same weight.
Condensation in a bivy will be horrendous under all but optimal conditions. Never use a down bag in a bivy. Here is my homemade bivy setup for typical desert conditions (no rain): http://frankrevelo.com/hiking/sewing_bivy_open.jpg