So multiattack only works if you have three or more natural attacks (basically you can't use the feat when you are in a form that has two natural attacks).
Elementals only have two natural attacks (two slams) but you can give them a third natural attack with the ring of rat fangs (a bite) that lets you then use multiattack as an elemental.
Mostly its just so that my monk/druid can do his (non-flurry) iterative unarmed attacks and then bite, slam, slam with a -2 penalty instead of a -5 penalty.
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