Are we discussing the fact that Bitcoin is not perfectly fungible, or how big an issue a lack of fungibility is?
Adam Back considers that Bitcoin is not perfectly fungible when he says:
some of the analysis you talked about could potentially lead to some currency units which you might receive for no fault of your own being somewhat tainted or frowned upon or, you know, so things are generally okay at the moment and the fact of it's been treated as fungible, but there is a slight risk that could degrade at some point.
Here's the full transcript of an interview with him about Confidential Transactions:
https://www.weusecoins.com/adam-back-confidential-transactions/Having read this thread I am actually coming around to the idea that fungibility is not as big a problem as I'd thought before.
I think the point some are trying to make is that since people (peers or third-parties) could trace historical movements of bitcoins, they could refuse them based on this history. It is a possibility, that's all. Whether in a world where Bitcoin exists alongside FIAT or not, it is still possible.
More Adam Back:
I mean, it increases indirectly so it doesn't directly -- I mean, so what bitcoin does have is a way to improve fungibility somewhat is the idea that the addresses are not reused. So, you know, when you make a payment and it split into a payment and change address the some ambiguity as to which was the payment and which was the change. And as that flows through the system and, you know, there are thousands of transactions in a big graphic it becomes increasingly not ambiguous as to, you know, it was --
So there are ways to improve fungibility, therefore it cannot be perfect in its current state. Regardless of how uneconomically viable you may think it is to exploit this imperfection, it is impossible to know how and who will exploit this in the future. Businesses might not survive by following such practices, but is it so impossible to imagine that government probably would exploit if it gives them greater control of people or trade? I'd say that's a distinct possibility, and this is my biggest worry.
Surely the best solution is perfect privacy (unlinkable, untraceable transactions), providing perfect fungibility. The best of both worlds. Why compromise?