I think it is a matter concerning the fishkeeper's aesthetic preferences rather than those of the fish. By the limited experience with my lot, the bare bottom does not seem to intimidate them one jot, never seen them trying to hide in a corner. This probably applies to most domestic hybrid discus as probably for dozens of generations they have been bred and raised in BB tanks. With wilds it may be different. The natural look is more pleasing and may well seem what most resembles the environment from which the species originates but for me this does not automatically imply that trying to mimick it in a tiny glass box is going to produce the best results. I have been growing out my discus for the last 15 months and in a few months, once I can relax the maintenance I was thinking of sprucing up the tank, add sime minimal decor and plants, some shoaling tetras and some sterbay Corys so debating weather to put a thin layer of pale sand. But plenty of people have reported that their Coreys do just fine on a BB