When I first got my chickens, I wasn't sure if I would like having them so I put heavy plastic down to protect the gravel base and make it easy to clean up when the coop got converted back into a storage area. (I lived next door to a commercial egg place as a kid and thought chickens were kinda nasty! Surprise!) When I quickly fell in love with my humanely-raised chickens, full of personality, I cleaned the coop and removed the plastic. It was NASTY down by the bottom! The moisture gathered and the bedding rotted, rather than composting. My bedding is so nice now on dirt with the gravel base offering great drainage, although it is not really needed, occasionally a scuffle leads to spilled water.
I did put a two-foot strip of vinyl flooring along the lower part of my walls for fear they would get composted with the bedding, though. It was salvaged from a flooring store, happy to give it to me for free, scrap from a job.