Able to replace black hole at SS (62 OPS+ from Jack Wilson since the trade, 68 OPS+ from Josh) with a decent hitter, which as you say is like replacing a decent player with a superstar. This is the same reason I would look hard at an offensive catcher and would find out about raising payroll to add a V-Mart. In his down year he posted an OPS+ of 122. Moore was 44, Johnson 63 and Bard 77. If you could get me Nakajima and just a 90 OPS+ at SS and 110 out of V-Mart we'd be adding a ton of runs.
Tony Blengino emphasized this a lot after 2009 -- "hey, how hard was it, really."
Harder than he thought :- ) as it turns out, but still. The M's have the easiest job in the world: upgrading a historically-bad offense.
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If you added Nakaji and V-Mart, you'd get not only the 25 runs from each position ... but IMHO you'd probably get a synergy effect, too.
When a pitcher has a tough fight on his hands, 7-9, rather than a breeze at 7-9, that's got to help the 1-6 hitters, right?