Now, refresh my memory - why do you care exactly which bitcoin gets spent? I mean, if I have a bunch of dollar bills in my physical wallet, I don't select one based on its serial number, I just pick one and use it.
You might care if you got the dollar bill from somebody you suspect is working for the Secret Police, you think think the Secret Police might be trying to trace payments by recording bill serial numbers, and you're spending the dollar to pay a local print shop to produce some illegal Subversive Propaganda.
That's roughly similar to tracing bitcoin transactions.