Now, I am a fan of the reusable grocery sack, and countless other items that are just plain better made with something besides plastic. But in some cases, a sheet of plastic can't be beat. Below are two photos of concrete floors.
The photos were both taken within a few minutes of each other, within a few yards of each other. Both are indoors, the left on in our shop, the right in the new apartment-in-progress. What's the difference? A six-mil sheet of plastic under the concrete on the right. It rained a lot the past weekend, then turned quite warm and humid. I can't explain the physics of it, but the damp in the soil under the slabs is sweating up through the concrete without a vapor barrier under it, while the other concrete stays dry. Conclusion: use plastic wisely, in the right places.