Problem with Singaporean reefers using DSB is due to the fact that most of us lack infauna and our LFS do not have recharge kits for us (currently culturing my own). Without these guys, our nitrates will just accumulate and render the sandbed useless. Furthermore, improperly set up DSBs will create a whole mass of anaerobic zone and black patches of hydrogen sulphide will start to appear.
But back to the topic, using LR solely for biological filtration is not enough.