Seth, how can you or anyone say that Plouffe was not a weak spot given his production (or lack thereof in 2013)? If every team in MLB had such poor figures from any position offensively they would easily be candidates for 90+ losses each year. And, guess what, that is also exactly the Twins victory total each year for the last three seasons.
Plouffe has had a positive WAR two years in a row, so, by definition, he was not a weak spot, where you define "weak spot" as at or below replacement level. The Twins had two positions with negative WAR last year, which they have not addressed. Yet, people are obsessed with a position with positive WAR.
And is he that weak? He is closer to average at his position than replacement level. His OPS+ was 106 and 94 in his first two seasons at the hot corner. So he's teetered on either side of average at his position (100 is average). If you want a good comp, consider Garret Atkins, who had a career OPS+ of 100. He wasn't as good a defender as Plouffe, but few people thought to get rid of Atkins after his first full season at the position with an OPS+ of 93.
BTW, in his next three seasons, Atkins proceeded to give the Rockies almost 10 WAR. So it's not out of the question for Plouffe to progress this year. People forget that this is just his second full season at the position. At 27, he's entering his prime. I expect progression. Getting rid of him now would be selling low. After going through David Ortiz nontender, I don't think Ryan wants to make that mistake again.
Is there room for improvement at third? Sure. Not in the current Twins organization at this time. Could they have upgraded the position externally? Doubtful. Besides, why upgrade a position externally that you plan on upgrading internally within the next season? Anyway, you don't get rid of an average player at a position who's cheap without a better backfill waiting in the wings.