Commit messages are immutable. Linking them to a bug ticket gives you a mutable place to record any new information about the bug that you discover in the future. (For example, that it affected more cases than you originally thought, or that the fix caused another bug.) This new information will be discoverable when starting from the original commit (found e.g. by doing a blame on a particular line of source).

To fail to do so is a gigantic missed opportunity in my opinion. You never know when you will need it.