One alternative to doing this, which came up in a round-table discussion between myself and Dave Cameron at FanGraphs on this subject, is to instead choose players based on past calendar-year performance. The advantage here is that it's a bit more straightforward: assuming we can properly measure everything that happens on the field (that's a big if!), all a player needs to do is produce better than the other players in the league at his position to get the nod. We don't have to worry about aging curves and regression and luck. Just produce, and you'll be rewarded.
Here's a comparison for how the three approaches would differ (and be similar), comparing my projection-based roster to one according to FanGraphs' WAR Calendar-Year splits, as well as Vince Caramela's first-half All Star selections.
|Position||True Talent All Star||Past-162 G All Star||First-Half All Star|
|C||Joe Mauer||Victor Martinez||Alex Avila|
|1B||Miguel Cabrera||Adrian Gonzalez||Adrian Gonzalez|
|2B||Ben Zobrist||Ian Kinsler||Howard Kendrick|
|3B||Evan Longoria||T-Adrian Beltre & Evan Longoria||Alex Rodriguez|
|SS||Yunel Escobar||Alexei Ramirez||J.J. Hardy|
|LF||Josh Hamilton||T-Brett Gardner & Josh Hamilton||Josh Hamilton|
|CF||Curtis Granderson||Curtis Granderson||Curtis Granderson|
|RF||Jose Bautista||Jose Bautista||Jose Bautista|
|SP||Felix Hernandez||Justin Verlander||Justin Verlander|
|RP||Mariano Rivera||Jonathan Papelbon||Jonathan Papelbon|
On the other hand, we do see some putative small-sample selections in the first-half All Star picks. Alex Avila's an interesting player and has had a great first half, but he ranks 7th by projections and is in a tie for second by the past-162 splits. Howard Kendricks' even more extreme, ranking 11th by projections (behind even guys like Nishioka and Jason Kipnis) and 4th by past-162 game WAR.
|Position||True Talent All Star||Past-162 G All Star||First Hall All Star|
|C||Brian McCann||Brian McCann||Brian McCann|
|1B||Albert Pujols||Joey Votto||Prince Fielder|
|2B||Chase Utley||Rickie Weeks||Rickie Weeks|
|3B||Ryan Zimmerman||Chase Headley||Ty Wigginton|
|SS||Troy Tulowitzki||Troy Tulowitzki||Jose Reyes|
|LF||Matt Holliday||Matt Holliday||Matt Holliday|
|CF||Andres Torres||Michael Bourn||Matt Kemp|
|RF||Jayson Werth||Mike Stanton||Lance Berkman|
|SP||Josh Johnson||Roy Halladay||Roy Halladay|
|RP||Mike Adams||Carlos Marmol||Craig Kimbrel|
Among the first-half All Stars, the biggest outlier pick is Ty Wigginton, but Chase Headley would be my selection based on the extra playing time this season if nothing else. You can also complain about Lance Berkman, although he was effectively in a tie with Jason Werth for the projection's choice...and a lot of his merit comes down to how good (or bad) you think his fielding is.
But insisting on using first-half statistics to justify an all-star game roster? There, I just don't get it.