Tracking a specific Bitcoin to make it represent something else, which could then be traded without trust
As you say following the outputs of a transaction could be used as you describe, there are more people that like that kind of use cases and that's why ColoredCoins was created or Zerocoin which try to make it completely anonymous.

Indeed so many people is trying to add metadata to coins that in 0.9 version of the client there is a new output type that can include 80 bytes of data with no other use (you can read more about it here https://bitcoinfoundation.org/blog/?p=290)