Military societies are not known for throwing away military advantages.
The Sith Empire is not a true military society. It's not rational, the very basis of the Sith Code is irrationality - passion doesn't usually lead to clear rational choices. It's leadership has no interest in the good of the empire, they are only concerned with their own personal long term struggles for power.
The Empire's reasons for signing the Treaty of Coruscant have nothing to do with military advantages or disadvantages but the insane whims of their emperor. The slave revolts on Dromund Kaas aren't due to a lack of power or control, but are a direct manipulation by some Sith to undermine other Sith.