[dgreensp]

The point of "use strong" is to enable optimizations that wouldn't be possible otherwise, among other things. If you remove it in production, you won't get the optimizations.

"use strict" is used in production all the time. Google and Facebook use it. The Babel transpiler (formerly 6to5) adds it to your code by default.

It's one thing to get JS programmers to write their code in a certain way, but if the VM knows they are doing so, it's even more powerful.

[azakai]

I think there are 2 points - the one you said, to enable optimizations, and the other, to guide people to writing code that is easy to optimize.

I guess the question is, how much better can you do, over easy-to-optimize code. Or in other words, once something is valid in Strong Mode, how much do extra Strong Mode optimizations get you.

Such code should already be quite fast in current JS engines. The big, big difference is between the engine getting hard-to-optimize vs easy-to-optimize code. So I would bet that the bigger deal is the devtool aspect here. But, these are empirical questions.