Well you don't get things like free wifi or breakfast at BW for being diamond (unless the property offers it otherwise to all guests) as a diamond at BW which are two important things for me.

I was told at the BW I used to stay at in Chicago that I would get upgraded regardless of I was blue or diamond based on how much I stayed there, so some properties must upgrade based on how much you stay there. Worked out great for me since they purchased a building next door and kept floor 1 retail and made the floor 2 to 4 apts into nice suites with kitchens and would upgrade me to those if they were available. It is unfair though to diamonds who would stay there for the first time to not get upgraded over a blue who would get upgraded just because they are a frequent guest of that property and that is the only BW they stay at.

Not sure if a property can tell if someone is a diamond on stays or status match anyway. It is fair to give out upgrades to real diamonds first though. I wish Hilton and other chains would give out upgrades to their real diamonds over people who get the status based on credit cards or fast tracks.