While there are users who love Gnome and only use Gnome and there are users who love KDE and use KDE solely, there also are users who love to use both but have a single system.
So I thought it would be interesting to watch how it works if we install both Gnome and KDE at the same time. That is why I thought to share my experience here.
From my recent experience with 10.10 Maverick and KDE 4.6 from backport, apparently it seemed that there is no big issues when I tried both of them together. But when I tried to install some plasmoids from KDE package manager, I got error message. Although I am not aware whether or not we have a fix for such issues, but for a starter these things might be a problem.
Overall, although trying both will probably not give any big issues, but I will prefer using either of them in one system.