This instantly struck me as an answer to a complaint that appeared heavily on the rogue pass thread - archers and mobs using cleaves break rogue stealth with dumbluck hits.
Even if I'm running with 3 displacement greensteel items, I usually can't justify turning on displacement for regular travel and stealthing. The "displaced automatically if stealthing" ability here goes a long ways to mitigate getting knocked out of stealth by a random arrow or a cleave that wasn't even intended for you.
Granted this means that for 1-29 levels junk shots will still break your stealth.
If the displacement lingered for more than a second or two (say, 10 seconds), I imagine players will munchkin this by toggling stealth every time they catch a breath in a fight so as to keep displacement up.