Tommy,
Just to give my 2 cents worth... I had a similar conversation with a friend recently.
It is possible to have a tank without any gravel/sand in it, but is it the best environment for the fish (especially if you are keeping bottom-feeders).
I have always tried to create a natural as possible enviroment for the fish, and find out if they have sand/gravel/wood/rocks etc in their natural habitat and copy it.
I am sure that others will add to the comments and will mention that the gravel/sand helps with the biological cycle in the tank, but I am not that scientific minded (although I do agree with it).
Jazz_UK