From this site and many others, it seems to be universally recommended that a lithium ion battery not be discharged lower than 3.0 volts. Most actually recommend to stop discharging at around 3.3 volts as there is little usable energy beyond this.
However, every lithium ion PCB or BMS stops the discharging of a lithium battery or battery pack at a voltage of 2.7 or even as low as 2.4.
Here is an example:
In this protection circuit, it stops the discharge at 2.4 volts/cell.
The reasoning I am given for not over discharging the lithium ion batteries is that it will hasten their demise and increase the probability of catastrophic failure such as fire or explosion.
So why the low voltages on the protection circuits and are our lithium ion drills etc being allowed to go to this low a voltage?