The simple problem is that, when there is a fast part in a guitar solo - sweep picking for example - I can't make out what notes are being played!
And, if I had the tablature in front of me, I would know the notes, but the rhythm would be a mystery!
The only way I could clear it would be to slow the part down, leanr it that way, then speed up.
But, because I can't slow CDs down, this means that when I'm trying to figure out a solo by ear, I'm at a loss. I simply cannot DIFFERENTIATE between the notes in a fast piece.
For an example, listen to some of the sweep sound samples in this site. Now ask yourself, if you didn't have the tab in front of you, would you be able to tell which exact notes were being played and in what order? I definitely wouldn't.