Well, couldn't the same question be asked about using Python or Ruby within a server? It's scriptable, easier and faster to iterate with. Dynamic. You can execute Lua scripts in Redis. LuaJIT and other implementations can be incredibly fast and pretty fun to code in.

You can use the excellent fullmoon framework that takes care of a lot for you

https://github.com/pkulchenko/fullmoon

Then using lua is not much different than python/flask