In my opinion, the main benefits of a shallow sand bed are :
1) Appearance
2) Diversity : you can simply keep a larger variety of critters
In addition, the main benefits of a DSB, deep sand bed are :
3) Nitrate Reduction : the anaerobic bacteria that are required to complete the nitrogen cycle (nitrates to nitrogen gas) do not flourish in less than 4 inches of finer sand (depending on water circulation).
4) Lower Maintenance : as NM said, with good fauna in the bed, a good clean-up crew and good current the sand bed cleans itself.
The main negatives of a shallow sand bed are:
1) It is difficult to maintain the fauna needed to maintain your sand bed requiring you to perform a larger part of the maintenance.
2)Without proper maintenance your sand bed can become a nutrient sponge, absorbing detritus and debris which can cause all sorts of problems.
3) You still need to find a method for exporting Nitrates out of your tank. If you plan on regular water changes the nitrates issue is mute (as long as you don't harbor a buring desire to have your nitrates at 0) otherwise a refugium with macroalgaes harvested regularly can help with this.
In addition, the main negatives of a deep sand bed are :
1) Several folks have reported tank crashes after having DSB for over 4 or 5 years. No one has any real evidence that a well maintained DSB was to blame, It's more a suspicion. (This is a mute point for most of us as we all tend to upgrade sooner than 5 years anyway).
2) Appearance : while the top stays clean the sand aganst the glass can be unsightly (I personally think it looks cool).
I just finished setting up a new tank, I am using what I'll call a MSB (medium sand bed). My old tank has a Deep Sand Bed which I've had no problems with.
My new display tank has about 3 inches of live sand. I chose the depth based on an educated guestimate of what I thouight it would take to maintain enough fauna to maintain the appearance. Since I am relying on a refugium with a DSB and macro-algaes to handle the nitrates I did not feel it necessary to have a 4-6 inch bed in the display.
Good luck deciding on how best to do yours, there is no shortage of opinions on this which I'm sure you've discovered.