The thing about Mercury is that it doesn't obfuscate the history of the coins you receive, but rather swaps your history for someone else's history, and leaves no traces on the blockchain that this has happened.
Now that makes sense. This is both good and bad. As witcher said, it would help to make Bitcoin's transparency a less reiable source of information. Ideally. But realistically, if I swap my 'clean' history for a 'tainted' one, I may now be falling under other kind of trouble I would not have previously expected. Just imagine you used Mercury and all of the sudden you own the coin of which history is a large hack like Bitfinex. Now that is a BIG red flag on your back you have to get rid of. To me, Coin Joining and Mixing is definitely superior. While swapping history helps turning transparency into a less reliable source of information, Mixing and Coin Joining does much more than that.
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Regards,
PrivacyG