Fragmentation has a cost, because higher levels (Qt,Gtk,etc in this case) must use portability layers. However, staying in one project also has costs: much more discussion and politics.
Both Qt and Gtk has long had explicit support for different backends. Whether or not the user uses an X backend or a Wayland backend or a Mir backend should have minimal impact.
Mir and Wayland seem to be pretty related though. Same gaps that Wayland has (needing development time) also are with Mir AFAIK. So once fixed for Wayland, Mir will have it easy :P
Note that real Mir support is only planned for 14.10. At the moment it is just XMir, which is totally different from the what is going on with Wayland (goal is native, this is way more difficult). I have seen the development that is needed to get Wayland really working, Mir seems way behind on this though they probably can copy what was done for Wayland and pretend they did it :P
