So, I was just wondering if there are any advantages of using the normal Lua interpreter instead of the LuaJIT interpreter for anything?
PUC Lua (the "normal interpreter") is more stable, i.e. bugs are more frequently found in LuaJIT.
PUC Lua is smaller, i.e. it takes less memory.
PUC Lua is much easier to understand if you want to customize it.
PUC Lua has been ported to way more platforms and is easier to port.
Like @lhf said, LuaJIT does not support all of Lua 5.2 features yet.