If you use a 10Kg CO2 canister, the time between when pressure starts to drop (end of liquid phase) and the time the canister is really empty can be of up to months depending on your bps. During that time, canister pressure drops and working pressure increases. Through the needle valve, an increase in working pressure by 0.1 bar can significantely increase the bps. On the best single stage regulators, you'll get 10-30% increase in working pressure, 30% being the most common for most cheap aquatic regulators, even the Dennerle and Tunze ones (tested). That's 0.6 bars for a set 2 bar working pressure.
Instead of looking daily at your pressures to avoid fish gasping at surface and having to correct pressure for weeks when canister empties, a dual stage regulator gives me peace of mind.