We're also talking about legal purchases here mostly, as that's what I'd be using it for, like Amazon and whatever other useful sites allow for BitCoins. Even then though, it'd be tied to my name. So I still fail to see the usefulness of it.
ok example time..
you put some bitcoin into purse.io (service to give you amazon giftcards).
all the blockchain see's is 1R4nd0m4ddr355blahblahblah -> 14n0th3R4ddr355blahblahblah
so lets say authorities wanted to know what 1R4nd0m4ddr355blahblahblah bought.
they would need to search the whole internet and hope that owners of 14n0th3R4ddr355blahblahblah is made public, or they would need to contact every bitcoin related company and ask if they own 14n0th3R4ddr355blahblahblah.
if purse.io admitted they owned 14n0th3R4ddr355blahblahblah. then the authorities will know 1R4nd0m4ddr355blahblahblah paid purse.io
lets say you were dumb enough to use your real life email address with purse.io
authorities can demand the email address linked to the account deposit 14n0th3R4ddr355blahblahblah.
but.
lets say you gave purse a throw away email. just used long enough to get your giftcard number. and you used that same throw away email to download a online book. where you used tor/proxy
in short the authorities are screwed. and your anonymity is secured.
again anonymity is only as good as the persons ability to keep their mouth shut and not reveal too much to the public.
and lastly., if your only doing legit transactions, the authorities wont even bother you, they wont be able to show a valid reason to get a warrent to compell purse.io either. if your doing nothing dodgy you got nothing to worry about regarding authorities.. but its still worth remaining anonymous incase friends, relatives or other people want to spy on you