The purpose of a footprint in the current configuration of most tents, as I understand it, is to primarily protect the floor from wear and tear. In the past, a footprint, tarp, ground cloth, etc. was needed to give added protection against moisture. Often, however, tarps would gather the water and send it underneath the tent where it would settle and eventually work it's way in to the interior.

I use a footprint only if I'm interested in maintaining and protecting the floor of the tent. Footprints, which are made to be slightly smaller than the floor (so they don't drag water underneath), can be easily replaced, but the floor of your tent is most likely irreplaceable. The tents I've been diligent in using footprints with during their life show very little wear whereas those that I haven't been as diligent with show wearing away of the coatings, abrasion, dirt I can't get out, etc (important to know if you plan on reselling it). One of my treasured tents, an MSR Fusion 2 Titanium (only 100 were made and mine is #5 out of 100) with custom Easton Carbon Fiber poles doesn't hit the ground without a footprint.

Many tents, such as the Hubba Hubba, use the footprint to add to it's value in that you can use the fly, poles, and footprint to create a lighter weight shelter when the canopy (body of the tent) isn't needed.

A footprint won't add any warmth to the interior for winter camping. It will keep the bottom much dryer than if you set it directly on the snow though. The benefit here is that you can pack the wet footprint in a separate place allowing the tent to dry out faster. This is important especially if you are constently packing it going from camp to camp rather than having a basecamp. In the winter, gear tends to get wet and dry out much slower and packing a wet tent can make things pretty miserable.

I would recommend the use of a footprint only if your interested in protecting the tent. Modern factory taped floors of most tents don't need the extra water protection that a footprint, etc. offers. However, if your paying 300+ dollars on your new tent, this small investment is worth it to keep it in pristine condition.