How necessary this is, well, that's an open question. I mean, we used to pitch tents without footprints all the time, and I can't recall anything bad coming from it. Tents really are pretty fragile, and sun and UV exposure pose as much or greater risk to a tent than physical wear. Still, on rough, gritty ground, it's a way to protect your investment (as is pitching the tent in the shade and staking it out tightly so it can't thrash around).
But I don't think you need to buy a purpose-built footprint. A homemade one is fine. Your cheap tablecloth fits the bill nicely; I usually use polyethylene sheeting purchased from a hardware store. The trick is to make sure the footprint is cut so it's a little smaller than the tent floor. In other words, you don't want any of the footprint stuck out from under the tent. If it is, then any rain that falls will be trapped on the footprint and will roll under the tent.
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Filed To: Tents