I honestly don't understand the benefit of RequireJS. If I minify and concatenate all my files using UglifyJS and place the single resulting JS file just before the closing </body> tag, do I still need to use RequireJS? Besides adding 15KB to my page-size, what benefit does RequireJS offer that you cannot get with UglifyJS?
The main point on using requireJs is to have a sane way to modularize your code. Lets say you have an app with some thousand lines of code, you don't want to handle this all in one file. So if you start split it into different files you need a way to load all the single files in the right order. Sure you can create 20 script tags and make sure they are in the right order by yourself, but as your code grows you have to add more script tags and for every file you have to find the right place. This is what requireJs is good for.
Lets say you have a 3 script A.js depends on B.js which depends on C.js
with script tags it would looks like this:
<script src="C.js"></script>
<script src="B.js"></script>
<script src="A.js"></script>
//A.js
console.log('A' + B)
//B.js
B = 'B' + C
//C.js
C = 'C'
})
with requireJs
<script src="require.js" data-main="A.js"></script>
and in every script you declare the dependecies:
//A.js
require(['B.js'], function (B) {
console.log('A' + B)
})
//B.js
define(['C.js'], function (C) {
return 'B' + C
})
//C.js
define([], function () {
return 'C'
})
This is a very simple example but the point is that you don't have to care about the order of your modules as requireJs load the in the right order.
Back to your question the optimizer is more an add on, so if you have a small code base which fits in a single file and is still maintainable, just use uglifyJs. But if you want to split your code in modules requireJs is the way to go.