I agree whole heartily, been playing the series since the beginning. It's not Sam Fisher without him being voiced by Ironside.
I don't usually base my decision on buying a game based on such a thing but with his voice being different it's not Sam, I would of actually not bought the last Assassin's creed if I had know they had gone with a different voice actor for Altair.
I also agree with the changing of the name of the character if it must be voiced by Eric Johnson.
So in short.