Tarps are great (you can use a net screen hung from the ridgeline for bugs pretty easily); I camped happily under them for a number of years. Back then, an 8x10 tarp and groundcloth setup weighed about 2 pounds, including stakes, cord and some bug netting. A tarp and bivy combo went for two or two and a half pounds. (I went to the bivy after rain and snow blew in a couple of times.)

So why did I quit using a tarp? Because they came out with 2-pound tents. Easier to pitch (mostly), bug protection, and full-coverage fly, with no penalty in weight, made them too good to pass up.