Is there a benefit to using KDE applications while running KDE?
Integration with the rest of the Desktop Environment, i.e.: appearance, context menus, preferred applications, integration with other/related apps, desktop sound effects, & etc. Mostly fluff, IMHO. KDE/Qt-based apps should (mostly) work fine on GNOME as "stand-alone" apps, and vice versa
, minus perhaps some of the "fluff".