If you let the mob evade, you're stupid. good point though, I can see how wasting vanishing powder could be a huge deal, seeing as how it stacks up to 20 and costs <15c.
Any rogue who doesn't use cooldowns while grinding or questing either doesn't know how to maximize their xp/hr, or is afraid of getting killed in PvP because they can't play without cooldowns. this is common sense and really should not have been posted here.