From the text (p.57-58):
“…If successful, the character is behind his opponent. The opponent can now be attacked with a +2 bonus to the attack throw, and gains no benefit from his shield. Thieves and others eligible to backstab an opponent gain their usual +4 on the attack throw and bonus to damage…”
Now is this a very ambiguous way of saying “Thieves and others eligible to backstab an opponent do not gain this bonus…”. or does it mean that a thief or other classes eligible to backstab who succeeds in the proficiency throw will get the +2 acrobatics bonus and their +4 backstab bonus?
Yup. And at high levels, they can more or less pull it off every round. Kind of all the benefits of the various tricks to provide flanking or flat-footed, without all the bothersome tracking of position and placement.