i think what he means is will the dirt on the floor effect his plants throughout the room.. i dont think he is growing in the dirt floor..
that being said i would think that the biggest problem at first would be the bugs and critters in the dirt.. i would run a bug bomb a couple times before i started my grow or if you have been growing with no problems i guess no worries.
but the dust may get kicked up and around and stick to your buds late in flowering and can get thrown around with the fans which could be a bitch. it could also potentially get in your pumps and fan motors which could shorten the life of them in theory.
i would suggest getting a nice big tarp to lay down so that you can walk around and have your fans on without kicking up too much dirt.