Good question!

In the past Penguinbait's install-tools (system cloning) package created and enabled a small swap partition on the SD card and under those circumstances enabling virtual memory wouldn't be productive (being compressed and more CPU-intensive than accessing plain swap partition).

However I just ran "free" in terminal to see how my swap was doing (system cloned using "pbeasy"), and to my surprise I found out that swap (total/used/free) was a big fat zero.

In any case, 1) when using swap partition virtual memory isn't needed and 2) without swap partition virtual memory slows things down a little when actually used but it might be necessary unless you know for certain that you won't need it.