If you have 20% defense on a fire farm you will die, unless you rely on greens/purples, which would be less efficient than only needing reds... But to your point, I'm all for only getting what you need. I definitely feel like most people tunnel on obtaining certain stats and end up going overboard.
When it comes to tanks, it gets tricky... I personally build mine to be more than just taunt bots but you will definitely find those people that literally just want to stand there and taunt. I completely ignore defenses on my rad/ss because it doesn't NEED defense, I have hard capped resists to all besides cold, absorbs, heals, like you said, the defense isn't needed. A lot of people try to also get defense and it makes their character nothing more than a meat shield.
It's 3:41 am here in Hawaii, I'm stuck in a work call, pardon my wordy response.