Its hard for me to understand because I haven't seen / read it anywhere, and it certainly isn't true in D2:LoD.
It is absolutely true in Diablo 2 (in some cases), except for crafted items, which can be even better.
Slots where rares can be best:
-helm (rare circlets and etc)
-weapon (for melee, not really caster)
-gloves (usually for zons)
-rings (more for casters)
-belt (on occasion, does not influence looks anyway)
The only ones that actually influenced the look of a character as far as I know are armour, helm, weapon, shield.
So yes, in general a lot of people wore the same armour and shield. Weapons too (ebotd/greif zerk/etc). Many people wore shako.
Just because it's possible in Diablo 2 and 3 doesn't mean it's likely. It is much harder to find a great rare than a legendary (with half or more static good properties).
So yes, people will wear a lot of the same stuff again in Diablo 3. Hopefully crafting decent rares can prevent this some, and dyes should be helpful too.