Having the Gaim core and the user interface "split" means that someone can write a user interface using a different graphical toolkit. For example, a QT based front-end to Gaim instead of GTK. There is currently an ugly patch that rips out some old code and fully splits the core and UI to create a libgaim... this patch is used in the QTopia version of Gaim and in Proteus.

Proteus is a non-free IM program for OS-X. It's closed source, and the GPL prohibits them from linking directly to Gaim code. They get around this by using TCP to communicate with an open source Gaim-based "server." This server links with libgaim.