It doesn't matter if you AOE or not, arcane potency is effectively a 3% crit increase. The reason for the multiple procs is simply (and only) the high amount of hits.
While Potency does come out to be a 3% with most spells, this is not the case for AM spam. AM has a "double dip" type feature. Clearcast/Potency will proc when mana is consumed, which is at the beginning of Arcane Missile cast, and will last through the next spell to consume the clearcast. Thus granting Arcane Missile spam 30% crit increase for 10 missiles or 6% over all.
Our elemental shaman just got this a couple weeks ago, and loves it. He's also got a ~40% lightning crit, but the especially interesting uses aren't from LB spam. An EMed CL that procs LO (love those acronyms) will indeed shoot 2 proc bolts. Also, and especially, Magma totem builds charges, as does flametongue weapon, since both can crit. (That last bit won us an OOM-fest attrition battle in the arena against 2 paladins with an unexpected burst.)