the only disadvantage and it may only apply to certain people is the look, I am used to the look now and kinda like it more than the sane. as mentioned above, easier maintenance, allows better dispersed water flow, gives you more "tank" that is usable.
as for providing a natural environment, this depends on what kind of fish you want to keep, some live exclusively in the sand, some never see sand and some can do with both. but 4 glass walls is definitely not a natural environment so go for what you like.
as for the sand bed being alive, I was of this school once, and if you search back you will probably find I was one of the biggest proponents of sand beds on this board, but now I would never have one again unless I was setting up a specific tank for something that needs sand. my smaller tank is an example of this as I have a fighting conch in it and until he dies he will have sand, but my SPS tank is bare bottom and aside from some green hair algae is very sanitary. The problem with the life in the sand bed is that acording to the good Dr. Ron (who started this sand bed craze) we can only support 10% of the number and viriaty of "bugs" that it takes to make a sand bed work properly.
a good comparison for Bare bottom VS sand is my two tanks, both are hooked to the same water supply and the bare bottom tank is sanitary compared to the sand one. In the SPS tank there is what is left of a hair algae outbreak from when I removed the sand and restarted the tank, but it is only still there for two reasons, well one reason my laziness
anywhere I have manually removed it, it has not grown back but then again you can hardly see into that tank due to the Coraline algae growth on the glass (another point of my laziness of late) as Brad can attest to. now the other tank has some funky brown velvet algae, hair algae, cyno, ect and it has the sand bed. remember same sump connects both tanks so they receive the same filtration, additives, ect. and both are stocked with approximately the same amount of rock/gal. the only difference is flow and the substrate.
Don't get me wrong both tanks look nice (if you could see through the Coraline algae) but I think over all the bare bottom is a much cleaner tank.
Steve