I think there is a very strong performance argument to be made for having mutability here. Along the lines of https://www.google.com/url?q=https%3A%2F%2Fgroups.google.com%2Fforum%2F%23!searchin%2Fastropy-dev%2FPerformance%2420degradation%2420in%2420coordinates%2420operations%2Fastropy-dev%2FW_Ol4GaWGeo%2FgdcUFERzNCwJ