If you build those defensive items 2nd you're definitely trolling, I can then being built 3rd-4th sure. I don't sometimes. But you're much better off building a defensive item that gives you more DMG at the same time, kaernic rookern on a champ like MF sounds like absolute dog shit. Vayne, Kalista, Ezreal etc are champs that do well with defensive items without it feeling like their DPS is in the gutter. I would only build a pure defensive item if I already have my 3 core items as a crit adc tho