"...its not required to have shiny new gear..."

WHAT???? Now my life has no meaning!

Actually, what you describe is a trick I used in my early, broke days of backpacking (when I had to outfit my son and myself with whatever was left over after the house, the car, food, etc.) My first shelter was the Green Poly Tarp, but then I found a $10 one-person pup tent (about 24" high) that was a single wall. One night, I found out why the expensive tents had rainflys and, incidentally, why they call them "bathtub" floors. So, I invested in a $20 nylon tarp (the prefix "sil" had not been invented yet) and pitched it over the tent as a fly. It worked fine - though I quickly discovered that a Visqueen groundcloth was lighter than the tent and that the tarp worked just as well without the tent.

Happy Thanksgiving!