I think the value of a ground sheet under your tent [DIY or store bought] really depends on the surfaces you pitch your tent on and how much you use your tent. I have friends that bought the same tent as I did at the same time. Their tent is still nearly in perfect condition because they use it a handful of nights per year on prepared campgrounds with smooth surfaces for pitching a tent.
My tent is history because I camped in it for 4 months + in one year and for a couple months+ in the other years I used it. My camp sites were rarely prepared and smooth...much of the time I was camping in the desert or on the beach - both spots with sharp pokey things lying about.
Additionally if you camp a lot in unprepared sites there is often nothing to sit on so having a tent sized piece of nylon to sit on is very convenient.