From the OP: They send players down on the eve of an off day and delay the promotion or recall of a replacement until after the break. Ryan Fitzgerald, Mickey Gasper, David Festa, Eiberson Castellano, and Edouard Julien have all been subject to this strategy.
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Let's take a look at this "strategy":
Fitzgerald was sent down on the 22nd and Correa not activated until the 23rd. First, why keep Fitzgerald on the roster when he can go across town and play in St. Paul? Second, why activate Correa before he's needed on the 23rd, in case he has a setback from the injury (or has some fluke injury with a water bottle or something)?
Gasper's first demotion was on an off day, but they recalled Lewis the same day, so that doesn't fit the "strategy." Gasper's second demotion was also on an off day, but they didn't have any choice but to send him back down the next day -- he had been called up to be the 27th man in a double header.
First, Festa wasn't sent down on an off day. Second, he's a pitcher. If he's not going to make the next start, it makes strategic sense to send him down. They called up Gasper and had an extra position player for a day before activating Lopez in time for his start. Third, had they waited a day to send Festa in order to give him an extra day of service time/pay, they would taken the pay/service time from Gasper, so it's a wash for the budget.
Castellano was sent back to Philly during spring training. Not sure how this forwards the premise.
Julien's was demoted on an off day, but Castro was activated the same day. Doesn't fit the premise.
And as others have said, the travel plan makes no sense to send McCusker to Seattle, just to send him back home.
It's hard for me to see a nefarious plot here.