Lots of folks eschew tarps for tents because they fear they will be invaded by vast swarms of biting and stinging beasties of various ilks, but mostly I find the weight and (usually) the inconvenience/unreliability of zippers is not worth the relatively rare times that need arises.
I admit there are some spots where the hordes of sandflies or mozzies can be quite daunting (and March flies here in Oz are sometimes quite dreadful) but most things can’t sting or bite through well chosen clothing (or your sleeping bag), the critters arrive in great numbers every time you leave or enter your shelter anyway, and all you needed to carry really was a head net (which can also deter flies from bothering you and works while you are walking) and such a head net need only weigh 11 grams (!) as in this iteration from Sea to Summit: http://www.seatosummit.com.au/products/bug-protection/nano-mosquito-headnets/?ref=outdoor so I misdoubt the desirability of lugging around up to a kilogram of netting inside which you will always be killing sandflies, mozzies etc anyway.
That being said, I am working on Nano Noseeum mesh doors for my http://www.theultralighthiker.com/the-deer-hunters-tent/ which I will close without zippers and which (at .7 oz/yd2) will weigh only about an ounce or 30 grams (the mesh is available here: http://www.tiergear.com.au/11/online-shop/no-see-um-mesh) for such rare occasions as I find myself camped out in sandfly heaven, eg at the Grant Burn on the South Coast track Fiordland NZ: http://www.theultralighthiker.com/south-coast-track-fiordland-nz-waitutu-to-westies/) You can slowly eliminate the sandflies/mozzies which invade your (netted) tent by clapping your hands together to kill them as they circle below your suspended lantern of a night (http://www.theultralighthiker.com/mini-super-torch-a-weeks-light-weighs-50-grams/) – or you can carry a mini atomiser bottle of insect spray.
See also:
http://www.theultralighthiker.com/insects-can-ruin-a-camping-trip/
http://www.theultralighthiker.com/mozzie-nets/
http://www.theultralighthiker.com/leeches/
http://www.theultralighthiker.com/end-the-misery-of-insect-bites/
