So the drop rate on personal loot is like ~15-20% (anecdotal, but based on watching my own drops AND everyone else's in dungeons) at worst. It's VERY rare you kill a boss and nobody gets a drop, more common to see 2-3 people get it. Look at the loot tables on these mobs - they're effing huge. If you don't put it on personal the odds of you seeing a drop you can use are way lower than that, AND you can only get one, AND it could just drop for armor types that aren't even in the group. The odds of seeing the piece of loot you want are astronomically low either way right now (since they also added every stat combination basically, plus warforged, plus sockets/tertiary stats).
Even in a set group of 5, you're going to get exactly Boss# loot with group loot, and are much more likely to get like Boss+2-3 with personal, and it's all going to people that can theoretically use each piece. Yeah, some guy might get something someone else could've used and you can't trade it, but you were way less likely for that item to drop period if you were on group.