Well it was due to the fact that the tank I was getting was only ten inches tall and I didn't want to take up almost 1/3 of that space with substrate.
However I got a rather big surprise in the mail just now....instead of getting a pico 4 I got a nuvo 8 tank. http://innovative-marine.com/nuvo-aq...ano8-0202.html
Needless to say I'm not planning on giving it back when I got it for the price of the 4, so now the depth of the substrate is less of an issue and where to put it. While I would trust my current computer desk to hold 40 lbs I'm not trusting it with 80lbs or so.
Hmm maybe there is some way to brace the desk underneath the tank so it's more trustworthy.... Its not the surface that I don't trust its the possibility of side to side motion tearing the screws out of the particle board uprights supporting it.