The tarp under the tent thing is debatable about being a good or bad thing, like everything it has it's advantages and it's disadvantages.
I personally use a tarp under out soft floor camper, but only if it's on a very slight slope, when it rains water can actually get trapped in between the tarp and the tent floor
so I make sure if water does get in under the floor, it has somewhere to go.
Even without downpours, I've noticed small amounts of condensation in between the floor and the tarp, so I guess it's the lesser of two evils, having to brush crud off the floor or wipe a bit of moisture up, either way you don't get off scott free
I'm a bit of a neat freak, even whilst camping. I HATE dirt, grass getting inside. With a 4-year old in tow keeping crud outside is a bit of a challenge. I found that a smallish, 1m long hall runner inside and one outside the door traps as much as I think I'm going to. Works for us !