I had one under my tent when i was traveling long term. It's probably worth it if you are going out for a while because everything you own gets beaten to shit if you're setting up and repacking it every day for months on end. It's just a bit of extra protection. I found that having it was somewhat useful for other stuff too, e.g. when i just wanted to sit or lie down on something "clean" i could just put the groundsheet, or when i wanted a bit of extra rain cover in a storm but wasn't in a position to set up the full tent. That said, for these secondary uses it would actually have been more useful to have a larger tarp and then just fold it to ground sheet size for overnight. I think if i went again i'd consider a larger tarp, but there is a downside to those too - if the tarp is larger than your tent, then rain and other crap can fall on the tarp and get between your groundsheet and tent floor, which voids the protection a bit.
As always, no perfect solution, you just gotta figure out what works best for you.