On a resist set like elec/ a few points of defense wont make a noticable difference, especially because you can't have debuff resistance for -def, and a lot of foes throw that around. This can change when you can afford a lot of +defense things (mainly IO boni and a few pool powers). At the point where those debuffers will miss most of the time, the defense will help most of the time. So for a god mode tank, yes, get those defense boni. But to build up a resist tank they are mainly useless.