Why does this matter?
Drafting a player doesn't guarantee you get to keep him forever. For most players, the rules say that after four years of being signed, they become available to be selected by another franchise in the Rule 5 draft. This takes place on December 11, and has been the source of more woe than joy for the D-backs. We lost Dan Uggla to the Marlins that way, and would have lost Ender Inciarte in 2012, except the Phillies sent him back to us - part of the deal with a Rule 5 pick, is that a selected player then has to stay on the major-league roster all year. If he can't hack it, he has to be offered back to his original team.
However, players on a team's 40-man roster cannot be picked, so this is the time of year when teams look at their prospects that are eligible, and move to protect the ones they most want to keep. This often means tough decisions, because there are a limited number of spots, especially since during the off-season, the 60-day disabled list can no longer be used to "hide" injured players. This means 40-man slots need to be used for the likes of Bronson Arroyo, Patrick Corbin and Matt Reynolds, as well as all the veteran players signed by the team. As a result, it isn't possible to protect every prospect.
Movin' on up...
The five added two were a pair of position players, third-base prospect Brandon Drury and outfielder Socrates Brito, plus a trio of pitchers: lefty reliever Will Locante, plus right-handed starters A.J. Schugel and Kevin Munson. Munson was actually selected by the Phillies last December, but like Inciarte, couldn't stick on their major-league roster, and was returned to Arizona in mid-March. As James noted in the comments yesterday, Locante's role as a LOOGY probably makes him especially vulnerable, since it's probably the easiest spot on the 25-man roster to "hide" a Rule 5 pick. For an example, see our acquisition of Joe Paterson in 2010.
Brito is an interesting one, considering he has yet to face a single pitch above High-A. As a player aged under 19 when signed, we get an extra year of control, but that has still expired, even though Brito only turned 22 in September. He hit .293 as a 21-year-old with the Visalia Rawhide this year, and also stole 38 bases, so it seems the team feared someone might take a chance on him next month.
Falling down...
It wasn't all good news, however, with pitchers Mike Bolsinger and Charles Brewer designated for assignment to make room on the 40-man roster for the new arrivals. The team now has 10 days to trade or release them, or put them through waivers where they can be claimed, if wanted, by another club. Both have seen some major-league action, with Bolsinger making nine starts for the team last year, but Nick Piecoro notes that we have added a number of minor-league free agents, such as Gabriel Arias, Caleb Clay and Justin Marks, and it appears the team considered both men superfluous.
However, it may not necessarily be the end for them as Diamondbacks. All the other teams are going through similar roster crunches, so it's not impossible one or both might be able to sneak through waivers without being claimed. You can never really have too much pitching...
It's hard to say if we'll end up losing anyone. The only name among our leading prospects that both has enough service time to be eligible, and isn't protected by being on the 40-man, appears to be Zach Borenstein, an outfielder acquired from the Angels in the Tony Campana + Joe Thatcher trade. As a 23-year-old at AA/AAA, he had a line of .258/.320/.432; decent, but probably not good enough to stick on a major-league roster all year. We'll see what happens on December 11.