If you can't find a reason they're not used, it's unfair to say that such a reason exists.
I've been running a DSB for about 10 months (the same DSB that was in a 10G for 2 months is now in a 20 long+sump with added sand). It's great to finally see some other people with one! It is a fantastic system. There is a reason they are used in SW tanks, and that is that they are very good at processing excess nutrients into easily removeable substances through the water column (via water changes or crazy amounts of macroalgae).
A DSB in a planted tank does the same thing, except (once it's well established) these nutrients are removed by the plants rather than by WCs. The effect is little to no excess nutrients (I have none), and increased plant growth in non-nutrient rich substrates. The DSB itself establishes it's own nutrients by the breaking down of detritus within the substrate, allowing the formation of Nitrogen, Iron, Sulfate, and other chemicals that plants will absorb through their roots and leaves.
That being said, the key part is establishing the DSB. It takes a freaking long time. Think of it as cycling a tank, but waiting for all of that bacteria to establish itself through all of the sand, dying, forming different bacteria, dying, and forming different bacteria. And there's no stable way jumpstart to either of the last two layers!
+1 on the MTS, they'll only stay in the aerobic region, and really help with sifting the mulm into the substrate. I would hold off on adding more fish for at least 2 or 3 weeks if possible, you shouldn't have any trouble if you keep with your WCs, but a fully established DSB can handle that number of fish in this sized tank with something like quarterly 25% WCs. Doesn't it get crowded and look hectic with so many fish? Or do they shoal up pretty well?
In conclusion, Tom Barr is wrong
Edit: sorry, I didn't see that you already had MTS.