Some 4,395 games and counting later, it's obviously here to stay -- destined never to please everyone, yet always full of opportunities for situations made possible only by crossing over the league boundaries.
The kind of unique experience that only Interleague Play can bring was on display just last month at Wrigley Field, where a rare appearance on the North Side became a sort of comedic long goodbye.
When Derek Jeter stepped to the plate in the eighth inning with the Yankees trailing, it was to a farewell standing ovation at the Friendly Confines. After the Yankees tied it to send it to extras? Again, an ovation in the 10th. And, naturally, once more in the 12th -- his, ahem, third "final" at-bat at Wrigley.
"I kept hoping it was the last one, every time I went up there," said Jeter, the all-time Interleague leader in hits (362), runs scored (198) and games played (276). "I was hoping the game wouldn't take that long, but I told you, the way the fans have been treating me on the road this year -- it's been unbelievable."
That particular stop on Jeter's farewell tour -- which also has seen a visit to Miller Park and a final trip across town to Citi Field -- will go down as another memorable event that simply wouldn't have been possible without a little cross-league scheduling.
At 20 games per team now, Interleague Play still represents a relatively small segment (12.3 percent) of the schedule, but it has been in effect throughout the season since the beginning of 2013. With the Astros shifting to the American League last year to create balanced 15-team leagues, it had to happen -- there would have to be at least one Interleague matchup throughout the schedule, and sometimes three like there are this week.
In the second year of Interleague 2.0, many familiar storylines are playing out. Such as:
The AL still rules: For all the concern that AL teams lose a hitter half the time in Interleague Play, the Junior Circuit continues to win more than half the games. Before 2013, the AL had built a .525 winning percentage. After a 13th mythical Interleague title in 17 tries last year at 154-146 and a 70-63 lead this year to make it 2,303-2,092 through Tuesday, the AL's winning percentage has dropped to .524 (actually, a dip of .000730 from the 2012 total), but it remains well above the waterline.
Yanks on top: Leading the way for the AL, the Yankees are 10-7 and will finish their Interleague slate July 18-20 against the Reds. It's almost certain they'll remain the team with the all-time best winning percentage in Interleague play by the end of the year, currently at 189-130, for a .592 mark, with three to play.
But their old rivals the Red Sox are gaining, having gone 14-6 last year and 6-3 so far this year to stand at 181-131 (.580) with 11 more to play. Boston has passed the Angels (.577) and White Sox (.561) on the all-time list since Interleague 2.0 began, and the A's (.553) have moved into fifth.
Winning formula: It's not a bad thing at all to have a good mark against the other league. All of last year's postseason teams had winning Interleague records. Then again, the Giants went 7-8 in 2012 before earning the crown, so it's not necessarily a prerequisite.
In 2014, the Giants have become the winningest team in baseball early on with a little help from a 6-0 start against AL clubs, already matching their victory total from a year ago, when they went 6-14. The A's (3-0) and the Marlins (8-0) are the only two other teams unbeaten in Interleague Play thus far, but they all have a long way to go. No team has made it all the way through an Interleague schedule undefeated, with the '04 Cardinals going 11-1 (.917) and the '02 A's and the '06 Twins and Red Sox all going 16-2 (.889).
It all began with the Rangers hosting the Giants on June 12, 1997, and the first pitch thrown by left-hander Darren Oliver, whose 20-year pitching career ended just last year. Other than the World Series and the All-Star Game, the National League and the American League went their own ways from Opening Day through the league playoffs before then, so it was no small shift in scheduling that arrived that day.
This year, Interleague Play already has brought some banner moments, including a homecoming series for Angels phenom Mike Trout, playing in Philadelphia near his New Jersey home for the first time. Or, in that same series that saw Jeter feted, the Yankees saw prized new arm Masahiro Tanaka suffer the first loss of his MLB career against a Cubs team that had been struggling.
There's more Interleague intrigue to come, including a four-game Bay Bridge Series starting July 8 that will leave either the A's or the Giants, or probably both, with blots on their Interleague records.
And, eventually, with the Astros meeting the Mets at Citi Field in the final days of the regular season, another chapter of Interleague Play will be in the books, having played its part in another season.