The bottom line is that while Nadia is indeed expensive she also consolidates an awful lot of power into one spot.

Anyway, I do think there comes a point where fixating on roles can become rather counterproductive--it's important to have your all your bases covered, certainly, but that doesn't mean units have to be all defense or all offense. Gellius is great against uber units but otherwise a premium tank really only has value over a merely adequate tank if the difference in defenses allows you to get away with less healers. Otherwise you may as well throw in a blocker that can get the job done while providing more oomph.