If you want a look at the long-term consequences of a culture of pervasive custom UIs, look at the current state of Windows apps. Every single app looks different in some minor way for no discernible reason. The sum of them on the desktop adds up to visual noise and clutter that is needless. I realize the differences in look are not intentional: GTK here, .NET there, each with a different layout engine.

But the sad fact is, nobody seems to care about it. There is no standard UI on Windows anymore, and it drove me nuts.

My favorite whipping boy in this argument is Trillian: http://pcworld.hu/apix/0911/trillian_astra.jpg