I have spent many nights under a poncho/tarp, though rarely used a bivy. Built properly, the tarp shelter works very well. As Hikermor states, it is about keeping the wind and rain off, there is no insulation. The bivy can help you stay dry and provide a windproof layer over your bag or blanket, extending the range of temperatures you can sleep in. They do tend to serve as a vapor barrier and condense on the inside in humid conditions.