Don't know why everyone thinks Vargas, Rosario, and Polanco are MLB-ready offensively. They're not:
A+ (2013): 20.2% K, 9.6% BB, .48 BB/K, .813 OPS (125 games)
AA (2014): 16.8% K, 10.6% BB, .63 BB/K, .832 OPS (97 games)
MLB (2014-15): 26.9% K, 5.5% BB, .20 BB/K, .734 OPS (82 games)
AAA (2015): 24.2% K, 9.1% BB, .38 BB/K, 1.158 OPS (8 games)
Notice how much worse his plate discipline numbers were in the majors compared to his last two seasons in the minors, especially his strikeout-rate. Clearly, he wasn't MLB ready. Also, notice how his plate discipline numbers are worse in AAA than they were in his last two seasons in the minors. This is evidence that he needs a little more time in Rochester. Big point: we might think of Vargas as a "home run or strikeout" kind of hitter because that's what he has seemed like for the Twins, but he was much more of a pure hitter in A+ and AA. If we want that kind of player--and we do--he could use more time in the minors.
Yes, I'm aware of Vargas's silly OPS so far in AAA. But in just an 8 game sample, that number is incredibly volatile--much more so than his plate discipline numbers. Vargas's numbers in AAA are being boosted by a completely unsustainable .421 BABIP.
A (2012): 16.1% K, 7.2% BB, .45 BB/K, .835 OPS (95 games)
A+ (2013): 12.6% K, 7.4% BB, .59 BB/K, .903 OPS (52 games)
AA (2013-2014): 20.8% K, 5.9% BB, .28 BB/K, .706 OPS (149 games)
AAA (2015): 17.0% K, 5.0% BB, .29 BB/K, .659 OPS (23 games)
MLB (2015): 30.2% K, 1.9% BB, .06 BB/K, .708 OPS (16 games)
Rosario was a stud hitter in A and A+, but since then his strikeout rate has increased and his walk rate has decreased. While both of these were marginal changes, together they've led to a pretty significant drop in his BB/K ratio in both AA and AAA; this is a large reason why his OPS numbers have been so poor the last two seasons. This development has been taken to an extreme so far with the Twins, as Rosario is striking out almost twice as frequently as in his minor league career and almost never drawing a walk. For a guy who really struggled in AA a year ago, and was not great (by his standards) in 23 games in AAA, it seems clear to me that Rosario could use some time in AAA.
Also, just for the sake of comparison, Aaron HIcks has struck out in 29.2% of his at bats so far in his ML career. Rosario has struck out in 32% of his at bats. Yes, it's early. But still not what you want to see.
A (2013): 11.3% K, 8.0% BB, .71 BB/K, .813 OPS (115 games)
A+ (2014): 13.9% K, 10.6% BB, .77 BB/K, .780 OPS (94 games)
AA (2014-15): 16.3% K, 5.5% BB, .34 BB/K, .730 OPS (79 games)
Once again, Polanco's plate discipline numbers are significantly worse in AA than they were in A and A+. While his strikeout rate is still quite good--albeit higher than in A or A+--his walk rate is significantly worse. As a result, he's drawing fewer than half as many walks per strikeout. His OPS is struggling as a result.
In short, it doesn't seem like Polanco has really mastered AA pitching quite yet. What makes us think that he's ready for MLB pitchers?
One last thing: what do Aaron Hicks, Oswaldo Arcia, and Kennys Vargas all have in common?
Answer: they all tore up AA pitching, skipped AAA, and then struggled in the majors to the point where they had to be demoted to AAA. To a lesser extent, this is happening to Eddie Rosario. Do we want to keep on having our top hitting prospects skip AAA?