Pretty old post, but i go in.
Short answer: Yes, of course. But i really would recommend to keep your scripts winthin the range of 250 lines.
So, to stick with that, the answer is now.
What i usually do is building a class for it, and require each needed files to it. Require, NOT EXTEND. I don't want to load scripts, that are absolutely not needed for this particular call. And yes, parsing also not really narrows your performance.
But why for jesus nails do you want to do that?
registration and login are two different things.