Gear in Diablo 3 is intended to be an interesting choice.
Do you opt for survival gear or make your character a ‘glass cannon’?
Do you choose class buffs or take a gamble and stack Magic Find gear, rewarding you with more rare items if you are successful?
The current trend I see of people switching to their Magic Find set when the boss is down to a low % life seems to me to be a circumvention of the risk/reward curve. You are receiving the benefits of Magic Find without sacrificing any damage or survivability stats.
I know that this is a staple of Roguelikes and I’m going to be unpopular for saying this, but it doesn’t make sense to me from an immersion or gameplay perspective for Blizzard to not stop players switching gear mid-fight.