September starts this weekend, bringing with it one of the things that makes baseball markedly different from every other professional sport on the planet: starting on Saturday, rosters are permitted to expand from 25 active players to 40.

This rule seems to be almost universally hated (except, one would think, by the extra guys who actually get called up rather than sloughing off home or to a fall league after their minor league season wraps up). Joel Sherman is the latest to pile on, noting that teams in September often face off with wildly different-sized rosters, his entire post dripping with something like incredulity that this sort of injustice is perpetuated in 2012.

I’m no fan of the expanded rosters, but I think this (very common) line of reasoning goes much too far. In answer to Sherman’s question, “even if it is 33 [players] vs. 29 [in any given September game], how is that fair?,” I’d offer that every team is equally free to promote up to fifteen extra players, and that unless its Triple-A team bus got into a wicked crash with its Double-A team bus, the fact that the second team had four fewer players available than the first was entirely a product of its own choice. It’s almost perfectly fair, really; if you don’t want to be short-handed against any other team, you’re perfectly free to call up more players of your own.

The argument (also put forth by Sherman) that it completely changes the game from the way it’s been played over the preceding five months, on the other hand, is pretty legit. Suddenly being able to use a pinch-runner or a lefty reliever without any noticeable impact on the depth of your bench or bullpen seems, in a lot of ways, contrary to what managing a baseball game is supposed to be about. That’s why I dislike the rule — not because it’s “unfair,” but because it unjustifiably makes the game very different.

Even that drawback, though, can be seriously overblown. The whole concept of the replacement players is built around the idea that there’s a whole universe of guys out there you could pull out of Triple-A and plug into a lineup, and that they’d get the job done…but not very well. That also defines your typical September callup; whether they’re prospects (usually not the “prized” variety at any rate) who aren’t quite ready or minor league vets called up to put on a uniform and continue breathing and circulating blood, most of the players don’t tend to be anything resembling difference makers. It might sound like a huge benefit, to Sherman and to the anonymous execs he spoke to, to have a nearly endless bullpen late in the season, but if the vast majority of the arms in the ‘pen are guys with around a six-runs-per-nine talent level who you’d never trust in any kind of meaningful situation anyway, does it really matter all that much? I tend to doubt it. It keeps arms rested in case of a blowout in one direction or the other, but that’s about it. By and large, in your typical roster-expanding situation, we’re talking about very few ways in which one or two games might occasionally be affected.

The exception, though — the thing that makes this whole time really interesting, or really infuriating — is that rare occasion when a really exceptional player gets called up, or when an ordinary player gets called up and does exceptionally well. The big story around this September 1 is that the Rangers are set to call up Jurickson Profar for the pennant push. But while Profar — a top-ten prospect by any reputable system entering 2012 — is a terribly exciting player who performed admirably at Double-A this season (much more than admirably for someone who played the whole season at 19) and will almost certainly be a brilliant player some day soon, his .281/.368/.452 at Frisco doesn’t fit the profile of a guy who is likely to immediately dominate the big leagues, and in fact, he’s expected to be used almost exclusively as a sub.

It does happen, though. In 1974, for instance, the Red Sox called up 22-year-old Fred Lynn on September 5 and gave him fifteen games, including most of the starts from September 14 on out. The Sox slipped from a half game in front on the day of Lynn’s debut to a seven-games-out, third-place finish, but it was no fault of Lynn’s, who hit .419/.490/.698 in 51 plate appearances and set the stage for his Rookie of the Year-, MVP- and American League pennant-winning showing in ’75.

Perhaps the best example is 1980, when the Phillies called up a 22-year-old pitcher named Marty Bystrom (you can also read about it here). His debut came with a mop-up ninth inning in a loss to the Dodgers, which put Philadelphia a full game behind the Expos in the NL East. Three days later, though, Bystrom threw a five-hit shutout against the Mets. He started four more games, pitched well in at least three of them, and picked up the “win” in all four, finishing his inaugural MLB season with a 5-0 record, a 1.50 ERA in 36.0 innings, 21 strikeouts against nine walks and one home run allowed. The Phillies finished the year a single game ahead of Montreal, and without Bystrom (who Baseball-Reference has as having been worth 1.7 WAR in just those six appearances), there’s almost no chance they’d have pulled out the division win and, ultimately, the world championship.

Then, of course, there’s the loophole allowing players who were playoff-eligible but are put on the DL to be replaced with another, otherwise ineligible player. The Angels were the first to noticeably exploit this, enabling a 20-year-old Francisco Rodriguez to carry them through the late innings of the playoffs and straight through the World Series; the Rays did virtually the same thing (minus the ring) in 2008 with that David Price kid, and may be preparing to do it again (though it’s much less clear who they’d do it with).

So as an entire system, then, I think the roster expansion is a slightly obnoxious and silly thing, of which much too much is often made (as by Sherman on Friday); the ability to add 15 replacement-level players to the roster just isn’t that big of a change or an advantage, especially when everyone has an equal right to do it. But every now and then, that one special player, or ordinary player in a special moment, sneaks through and changes the whole complexion of a team’s season, or a league’s. And that’s worth watching, and getting excited and/or infuriated about. It’s hard to come up with a player that’s likely to fill that role this season, but then, so few things like this seemed “likely” before the fact.