The issue I have with client-side decorations is that presumably there's no (easy, universal) way to turn them off. My window manager is keyboard based, and I enjoy not having to waste my screen real-estate with large titlebars and buttons that I don't need. Not having a way to turn them off easily would be a pain. But maybe that's something the toolkits will just add as an option in .gtkrc or equivalent, in which case I cease to care.