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All this is true, but I'd add that Aardsma low Hr/Fb% are somewhat repeatable (low career rate and also in Safeco) and his strange LH/RH splits make him a good candidate for the park as well.
Considering the defense in addition I'd bet on an ERA in the mid to high 3s. I'd definetly trade him if someone valued him like a quality closer, but its looking like another Jose Lopez situation here..
As you said hes fine as middle reliever, but I don't see him as an elite bullpen arm.