bit-perfect is an audio capture of a digital track where the actual track is transferred verbatim (read: identical copy), bit by bit; there are some (many) capture cards that change the signal somehow, even using a digital input...
So, whenever is possible, a bit-perfect capture is preferable; but a non-bit-perfect is usually very good too... I must add that also an high quality capture of the analog laserdisc track stands very close, if not better sometimes!