An underestimated pitching style
I chose these players because they all fit into the category of underestimated middle relievers. Combined these 5 have pitched almost 200 innings and allowed only 44 total earned runs. This means their combined ERA is a mere 2.04 this would be second best in the majors if they were one player. This raises the question: what makes these pitchers so undervalued?
One reason why these players fly under the radar is because they are lefty/righty specialists and this job is not seen as one of the highest importance. They are also put down with the argument that they only have good stats because they are only matched up with batters they can do well against, and if they were pitched against batters from the other side their stats would suffer. This is true however an out is an out and as long as there are enough lefties to make it worthwhile to have someone to get them out than lefty specialists are very helpful to success in tight games (same goes for righties). The five pitchers mentioned above against their preferred side of the plate have kept opponents to a .184 OBP and a .233 SLG which are both incredibly low. To give some perspective, this is much much better than the very best pitchers in these categories at overall batting against (OBP: Scherzer has .225 against, and SLG: Greinke has .262 against). Although perhaps the job of bullpen lefty/righty specialist isn't quite as important as a starter because these 5 have also only averaged about 39 innings pitched so far this season whereas the average starter is in the 100+ range by now, they are so good at their job it outweighs the small amount less important that this job is.
There are also a few other smaller reasons why these pitchers are undervalued these include the fact that this pitching style is unconventional and the idea that it could cause injuries. Clearly it doesn't matter if a pitching style looks strange or is unconventional as long as it works, and I have yet to see any evidence in DL times of sidearm pitchers that supports the idea that sidewinders get hurt more, but here are some facts suggesting that they aren't hurt more: Joe Smith has averaged about 58 IP per season over the past 9 seasons. Javier Lopez survived 5 full seasons in the minors followed by 13 season in the majors, and is still strong at age 37. Brad Ziegler has averaged 67 IP per season in his eight year career and his least in a season was 58.1.
The idea that sidewinders have unimportant roles on their teams or that they get hurt far more than other pitchers is wrong and clearly it doesn't matter that they're style is unconventional so we really should be giving these important bullpen pieces more credit for they're work.