Creating self contained libraries

[eluser]jedd[/eluser]
I guess it depends when and where you want your pain.

Upgrades to CI are much easier if you haven't mucked around with your system directories/files. Other CI programmers that may work with your software are likely to assume no changes have been made to system files, also. By 'other programmers' I'm including you in 6 months from now, too. OTOH, upgrades to CI aren't usually considered mandatory for extant sites.

Conversely, for your own change management, are you happy to test changes to your library across all 6 of your apps prior to pushing out changes to this one file? If so, I'd suggest symlinks.

A deployment script might give you the same time saving benefits as sticking the file into system/

I just can’t understand why you are saying to leave /system alone if this is an open source framework that in its own documentation explains how to extend core classes.

You'll doubtless have noted that the documentation explains how to extend core classes by not fiddling with the core files directly.