Can it be used to hack OTHER sites you visit? No.
It can be used to pick and choose which websites to hack, ex. man in the middle attack. From there you can capture encrypted data using wireshark if the website has an ssl certificate or unencrypted data which would allow the attacker to log into your account easily.
bitcointalk was hacked not too long ago.
Exactly, so if a hacker knows that he often goes to a poorly coded blog/porn/etc site, they can edit a code in the background to make him download a FUD RAT.