while that is one of the options, there are many more. One of these options is ease of transport. Look at what's happening in 1.8 with charged creepers, and you'll understand what I'm thinking. The heads will be far easier to get. Also, the mobs could be sold in shops, without having to use fishing rods or commands. Also, it can be used in pvp. Perhaps there could be a command to disable the placing of them on lots with say /setflag critterbottle. It also seems a little less cheaty as you cannot stack them, it would be a way to prevent spamming in pvp/causing too much server lag.