An interested user wrote to the Wayland mailing list to say that client side decoration support should be an absolute must. Basically this would allow individual programs to control the appearance, mapping, and functionality of its window(s) without necessarily being controlled by a window manager. The message in full can be read here.
Kristian Høgsberg, the creator of Wayland, shares a similar view on client-side decoration supports as well. Beyond giving applications more control, Kristian reminds us that under this model the entire window could be one texture so there can be bilinear filtering , etc, to improve the visual experience of the desktop.
The opposition to having client-side decorations in Wayland would that -- depending upon its actual implementation -- it may be harder to close or move hung/unresponsive applications. However, Kristian says those beliefs have been "blown out of proportion." As part of this latest round of mailing list talk, there's also been ideas proposed about other ways in addressing unresponsive applications running on Wayland.
Sam Spilsbury of Compiz on the other-hand, wrote this email as he's basically not in support of the current client-side decorations proposal. After reading that, there's another response by the thread's original author.
There was also another email where an individual wrote about his experiences with old windows systems and client side decorations. He went on to write how under his interpretation of it, there would be a bad user-experience and that server-side decorations are a better solution. This led to an explosion by Kristian over the miss-communication and FUD that comes up about client side decorations. There was also another blunt email in response to more Wayland FUD.