The reason people use LuaJIT instead of v8 is because LuaJIT is faster than v8 (on some code) and is smaller and is more easily embeddable. Or at least that is my impression, I have no personal experience with it.
For me the main reason was the FFI, even if back in the day it did not support re-entrance - e.g. something in the "C" land has to call back "lua" land.
But the JIT in luajit is simply too impressive to skip it over. I was able to quickly prototype things with it, running almost at "C" speed, and some times even faster.
