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Because none of the teams that did deals with Z felt like they were being fleeced at the time - they were getting what they wanted out of the deal.
But you're right that a solid MLB ready shortstop will be more costly and harder to come by than an MLB ready outfielder. That's why Z did the Jack Wilson deal in the first place.
Guti, Aardsma, Branyan, they weren't really secrets. Nobody expected Aardsma to gel like he did. Most GM's knew that Guti was a defensive whiz trapped behind Sizemore, and based on his age you would expect his bat to improve a bit.
Olson, French, Vargas, Cedeno, he didn't exactly do in fleecing to acquire those guys...
Wilson as a stopgap? you could do worse I s'pose