Multicast does have a similarity to preferred server in the fact that the distribution source files will be distributed at a local level but the primary reason for utilizing TMC is bandwidth savings. Using a Preferred Server allows you to distribute from the local subnet but the clients still individually copy the files locally. That means that a 100MB distribution to 100 machines on a single subnet will take 1GB of data to cross the infrastructure. In a TMC scenario a 100MB package to 100 machines will take 100MB of data to cross the infrastructure. This means that you will minimize the impact on the local network as well as hopefully deploying a large package more quickly. The multicast is a joint conversation between all targeted clients. It would be like the difference of being on a conference call verses calling each person individually.
As with any good thing there are some drawbacks. The deployment will only be as fast as the slowest link and the source files have to originate from the host share. Where as with a preferred Server scenario you can stage the source file locally to the target subnet. That's not to say you cant pre-stage the files on a MDR first.