The aura gets the bulk of it's benefit on the 1st rank--subsequent ranks give far less incremental benefit than the longer silence duration. It is a somewhat better farming skill than Silence, but if you were going with a farming build, you wouldn't have maxed Arrows first anyway (farming build would probably be something like 1-1-1+2xStats before 6, then taking more aura after 6 before leveling arrows/silence). It's not really logically consistent to max Arrows first but then max Aura afterward.
S&Y should be situational. I honestly buy it more often than Manta now, since after the various changes to Marksmanship, Drow Illusions don't get ANY of the Agility bonus from Marksmanship, making them incredibly weak.