@Steve
You can still get pin holes through silnylon – the dyneema floors used in Tarptents are of a much heavier type than the fly and is going to be stronger than the silnylon (although perhaps Henry can comment).
You are buying the Li because you want to the lightest possible, fully featured shelter. A 2 oz polycro footprint will give you many nights of protection (should you think you need it) and if you get a tear in it, you can recycle it. The cost might be a couple of dollars at max.
The dyneema floor used in the Li products is also much more waterproof than the silnylon. It has a high hydrostatic head and when you pitch on wet ground, it won’t permit water to soak through. Silnylon, at least in my experience, can in certain conditions – it also seems to lose its waterproofness over time.
Finally, with the dyneema floor, it won’t absorb anywhere near the water weight that silnylon will which means a lighter shelter to carry after a rainy evening.
I am carrying a Protrail Li right now (I also have 4 other shelters) and on a more recent trip a month ago I did bring a polycro footprint. It actually never came out of my pack as the ground that I pitched the tent on was free of any pointy things.
Just my 2c