Personally, I use a ground sheet just to keep the bottom of the tent clean and a bit drier if the ground is damp.
The only problem with polycro is it's so light and flimsy, any hint of a breeze sends it flying. Trying to get it to stay put where you want it until the tent is set up over it can be frustrating. It's also hard to see if it does get loose. You got to find a bunch of rocks. While the polycro is fairly robust, if a tear does start, there's no stopping it.
I'll stick with the slightly heavier Tyvek. If you make it a little oversized and put grommets in the corners, it can double as a small tarp.