It depends on.......

1) The weather.

2) The ground surface.

3) The moisture content of the ground.

4) The type of trip I'm doing. Light and fast=no footprint. Slow and easy=footprint.

If the ground is already dry but heavy rain looks imminent I may not use one. As Jim said, I don't need a big bowl to catch all that water. Even if the edges of the footprint are behind the drip line of the tent. Water will find its way in.

Silnylon tent floors are not truly waterproof. Try sitting or sleeping on silnylon placed over saturated ground. You'll see. So I usually use one under silnylon. Unless I'm going really light, in which case my mindset is to tolerate more discomfort.

I'll usually lay down a thick black plastic (Home Depot) sheet under my tent in the snow.