The id you send should identify the user in your system. The plugin will store a map of NodeBB user id to your id for synchronization.
Why requestjs over browserify?
So it looks like requestjs and browserify solve some of the same problems but in different ways. There are a few features that browserify seams to have that requestjs does not. For example, browserify can bundle all your dependencies into a single file (which you are already doing for most of your browser dependencies but for some reason using requestjs for others). But here is the kicker, browserify allows you to use nodejs core modules in the browser with its shims that give you access to the modules below. So would browserify be a good or bad fit for NodeBB?
To be honest? Only because I'd heard of require.js before, and it was my go-to choice. I hadn't heard of browserify until Require.js was already baked into the core code.
It might be tough to switch over to the other, I honestly have no idea. We have a lot of standalone plugins written to define as Require.js modules...