Couple of reasons you cannot use different names.
#1 If the express are setup to extend wireless they MUST use the same wireless name.. that is how extend wireless works.
#2 If the Express are setup with ethernet connection and all have different SSID then roaming and transfer from one AP to next will be even worse. The problem is Apple set the point where it begins searching for another AP too low and you cannot change it. By the time the signal actually drops out completely it has failed to work properly long before.
#3 Changing the SSID will take even longer for the phone or device to accomplish.
But I do suggest you give it a go.. I believe in testing stuff yourself. Wire a few of your Express by ethernet and use different SSID on all of them, then walk around and see if the phone switches easier between each AP.
The advantage of mesh systems or Ubiquiti type is several new standards that improve roaming in wireless AP.
Apple cancelled the whole thing before they got there.