The option used to be on by default, however to my knowledge it is now always off by default.
That is correct. If the 300-500 MB of RAM it will use isn't a burden, then there's no real need to have the option turned on.
Note that if your pagefile is on an SSD, you may not notice any performance differences between having the option on or off. It may also work differently on Windows 10, where Windows favors compressing memory pages rather than moving them to the pagefile to reduce memory usage.
The way it works, turning the option on would reduce performance, and turning the option off would improve performance. The text of the tooltip may need to be updated to clarify that it is off by default due to reduced system performance when the option is on.