Command blocks can be used as custom mob spawners with the 1.7 development snapshots!
You need mods to obtain mob spawners in Vanilla Minecraft. But now in the 1.7 snapshots, you can make a command block that acts just like a mob spawner!
Here's how to do it!
/give <playername> 137
to give yourself a Command Block. Then, place the Command Block and enter this as the command for it:
/summon Zombie ~0 ~2 ~0
Then connect a timer to the command block, and each time it receives a signal it will spawn a Zombie 2 blocks above the Command Block!
This can also be done using a Command Block Minecart as well. Enter that command, and have powered Activator Rails along the track, and each time the Minecart goes over the Activator Rail, a mob will spawn!
This is a good point, but i see it being implemented more in adventure maps then in a game. I mean, if your in Creative mode, you dont need a mob spawner, and if your in survival mode, you cant use cheats (Yes in know there are exceptions :P). Great idea though!