I use a groundsheet with my Six Moons Skyskape X. The bottom of the tent is more fragile and more expensive than a silnylon tent, so I feel it is worth it. One of the side benefits of using a groundsheet is that it makes setting up the tent much easier. I can lay out the ground sheet and see where things line up in terms of slope, wind, etc. My groundsheet is clear, so I can then pick out the pine cones, little rocks and other debris. But even with a Tyvek groundsheet, the ability to lay out your tent spot with a simple sheet (instead of a tent) is a really nice.
I also agree with the other benefit. When it comes time to move to the next spot, it is much easier to clean a groundsheet than it is a tent. Wiping it down is much easier and cleaning it at home is easier.
In general, depending on your situation, it is a luxury item, but one worth the weight (in my opinion).