This is based on my experiences in the Sierra and a summer where my presence seemed to guarantee rain. I ended up being very thankful for my choice of tent.
If you expect rain, and you are in the high, rocky, lumpy country finding a site that has the right slope, and drainage pattern and allows you to place your stakes can be a problem. At the very least a built in floor is going to make things easier. Some people build ditches to create the right drainage. That is unacceptable to me personally.
This summer even with my freestanding tent I had be very careful in picking my sites. I have two dogs who sleep under the vestibules, one on each side. The first trip one of my dogs had to abandon the vestibule because it flooded out in a heavy rain. That has made me very sensitive to the drainage issue.
That was one the big reasons I dropped using a tarp, that plus the amount of gear it took to protect my bag from rain and wet dogs, hold off mosquitoes and hold the tarp up (I don't carry poles). I ended up with more weight and less protection, plus the hassle having to find just the right spot. And then there is well known law of physics, the force of attraction between a wet dog and a dry down sleeping bag is insurmountable.