Also note that you act as if there was only one way to spec UB (auto-attack damage), which is blatantly wrong. If you are facing off against a Rook (which in itself, might be a clue to spec differently) and you notice he's going armor heavy - don't play into his game by simply doing what you always do. If you can't find a single-target spell-based spec for UB (something I doubt), you can easily switch tracks and go for an army killer build, circumventing the armor-heavy build of your opponent.
I'm all for flexibility. Adapting your build to circumstance and opponent choices is great. 
What worries me, in present circumstances, is that UB without armour doesn't kill an armoured opponent, regardless of build. Neither does QoT. Neither does a Minion-Sedna, a Fury-Regulus, or a Minion Oak.
Your response is that they shouldn't play those DGs or builds? Where is the flexibility there? What we end up with then is a few optimized builds and DGs which everyone will play, because that's what wins...