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The bug causes the phone to stop responding to input and freeze randomly, which can be quite tiresome. Along with fixing the bug the update also fixes some security issues.
AT&T users would probably prefer to get their hands on Android 4.1 Jelly Bean, which started rolling out Sprint's variant of the Galaxy S II, the Samsung Epic Touch 4G. Even though this isn't the case, an update which improves stability and security is always welcome.
source - AT&T | Android Police