On my MSR, I'm using a footprint, because I don't trust the flimsy floor material withstanding all the rough ground conditions I'm facing here.
The footprint (I'm using a piece of Tyvec-like housewrap) serves more purposes for me, like being a tarp while taking a short break in the rain, or as floor&bed when taking a nap in the sun.

My Exped I'm using for winter camping mostly, and there is no use for a footprint.

Footprint needs to be a few cm smaller that the tents outline, in order to not gather rain streaming down the fly.

Some people here at WB say, they never use a footprint and never had a problem.