MATE itself has no plans to use headerbars, perhaps GTK will force our hand at some future date but so long as that is not the case then the traditional look and feel will be preserved.
However, now that Ubuntu is embracing GNOME I expect many more applications to use headerbars. After all Unity 7 used traditional window controls, so many applications haven't adopted headerbars. That will certainly change. MATE and Ubuntu MATE can't control what other application developers choose to do and we may have to choose default applications for Ubuntu MATE that use headerbars in the future, if the best application for the job utilises them.
We can mitigate the impact by styling headerbars so they are not so damned massive