[cellarius]

The overhead of including a php file depends on the size and number of files, but for your average addon I'd say it's rather negligible.

Why do it? It is simply way more comfortable to develop addons using your favourite editor and just hit the save button over having to copy the code to the AdminCP input box every time.

The main benefit regarding performance is that you have to keep less code in the datastore cache (only the include vs. all of the actual code). Overall, I'd say the tradeoff is well worth it.