Aw, come on guys. You all are talking about why a tarp/bivy or whatever is just fine as far as you're concerned. That has nothing to do with the question, and if you are planning on taking you wife/girlfriend/daughter or whatever along, you really need to bury that machismo stuff and cater for your lady's needs. Three is no way to have a romantic snuggle when the bugs are biting and you're both in bivy bags with a headnet on. Hard for your gal to enjoy eating her dinner through a headnet too. Get real, and get a real tent for those trips (beg/borrow/steal/buy). And face up to the likelyhood you will also be the one carrying the extra weight.
OTOH, if you're going out with one of your male friends for some tough bonding, then feel free to use whatever your little heart desires.