Once it becomes relatively easy to verify whether a Bitcoin was stolen or not, legal pressure will form on exchanges and merchants to not accept them. In most countries, knowingly handling stolen goods is a crime. So step by step, we will get those blacklists in one form or another. And it is better to get them sooner and in a way we can shape - than to passively watch them appear.
(emphasis mine)
If you want your Bitcoins to remain fungible, not just not, but also in the future when a crime is detected
long after it was committed you are better off ensuring it
isn't possible to know where they came from.
I'm reminded a bit of John Dillon's
pull request, particularly the replies by Mark Karpeles (MtGox) and Gavin Andresen. Neither sees anonymity as valuable or something the Foundation should push for, particularly Mark, yet if you receive Bitcoins that you know are anonymous and/or know you can spend anonymously you have a very solid guarantee that you will be able to spend them later without having to worry that a theft may be discovered after the fact, retroactively causing them to be blacklisted.
If Bitcoin wants to be the electronic cash of the internet, it has to be possible to break the inherent lack of anonymity the blockchain implies, or a Bitcoin will never be as fungible as a dollar bill.