Some members only have the older heroes, so they are being use. However as members aquire the newer heroes they do tend to shelve the older one. Oan: I have always said AC was one of the most under rated heroes in the game.
I think it's a huge waste too and have long thought there should be special game modes that used them, like green-only or blue-only hero modes, etc. But IGG have had years to change their minds about this and have not, so it's safe to say heroes will be obsoleted whenever possible to encourage players to keep buying new ones.