I do the bivy thing in the winter. You're more exposed when doing camp stuff and spindrift can be a problem. There is no interim temperature space that the tent provides, you are outside or in your bag. The simplicity is nice and where you can set down for the night expands considerably. It's an acquired taste for most folks but is really just a very useful technique for certain types of walks. If you add a tarp or tarp tent you get an even more flexible set up but you begin to approach the weights of some of the lighter tent setups. There are two types of bivies: waterproof breathable useful as your only shelter but can frost on the insides and not water resistant- breathable which are lighter, frost less, but are more an over bag than a bivy. In the winter I'd favor the water resistant more breathable, but if you think you will be in rain with just the bivy waterproof is desirable. Water resistant ones are a useful piece of kit as they are light, can boost your bags rating 10°, are excellent in-tent frost barriers, good for emergency bring along on a day hike, useful in a snow cave, will protect you for a time from rain and are considerably cheaper than the waterproof ones.