The most common reason is that we ship our own version of the clearlooks theme engine to ensure that we control the appearance of GTK+ widgets, and we load our own GTK2 RC file to define colors etc. for those widgets.

This leads to 2 errors, one irritating, one that prevents the program from functioning.

Irritating: GTK+ loads the system GTK theme during a call to gtk_init(). If this theme is based on clearlooks but contains declarations unknown to our (older) version of Clearlooks, you get errors.

Blocking: GTK+ loads the system GTK theme during a call to gtk_init(). If the theme file defines a "color theme", then when Ardour defines its own, GTK will block for ever, because of a bug in GTK+2. This doesn't happen if the system GTK theme has no color theme definition.

Those are the two main issues. They cannot be worked around without rebuilding the GTK+ stack so that it cannot or will not load the system GTK theme.