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Update: in the comments, Dave has left a very thorough and convincing counter-argument that I think has changed my mind.
So one of the neat things I've realized recently is that I can save a lot of effort and mental energy by poaching post ideas from you guys. Just yesterday, in discussing the Kingdome and Safeco park effects, someone brought up the idea of changing Safeco's dimensions. I feel like this is an idea that comes up pretty often - at least every winter. And the idea tends to be that some people would like Safeco to be a little more hitter-friendly. Nobody wants it to be a bandbox, of course, but as it stands now, Safeco is among the more extreme pitcher-friendly ballparks in baseball, which does a number on run-scoring. And, after watching the team struggle to score runs at home, many people will suggest that the left and center fences be moved in a little bit to make it more fair.
It sounds nice at first, but upon deeper investigation, there are a couple issues. The first one is pretty obvious: anything that helps the hitters is going to hurt the pitchers. If they moved in the fences such that Safeco became, say, 2% more hitter-friendly, then you'd expect something like a 2% increase in offense, but you'd also expect something like a 2% decrease in run prevention. So nothing's really gained. Maybe you find higher-scoring games to be more watchable, but a lot of people think the opposite.
The second issue isn't so obvious. And this one has to do with home field advantage. What follows is a table showing every team's win percentage at home and on the road from 2002-2010 (this is as far back as ESPN's standings page would take me). I know a few teams have switched stadiums during this time period, but that isn't super important for our purposes.
| Rank | Team | Home Win% | Road Win% | HFA |
| 1 | Colorado | 56.8% | 38.7% | 18.2% |
| 2 | Tampa Bay | 52.6% | 37.9% | 14.7% |
| 3 | Pittsburgh | 48.0% | 35.4% | 12.6% |
| 4 | Washington | 50.3% | 38.2% | 12.1% |
| 5 | Texas | 55.6% | 43.5% | 12.1% |
| 6 | Minnesota | 61.0% | 49.0% | 12.0% |
| 7 | Houston | 57.0% | 45.0% | 12.0% |
| 8 | Seattle | 52.9% | 41.9% | 11.0% |
| 9 | Boston | 63.2% | 52.3% | 11.0% |
| 10 | Toronto | 55.2% | 44.6% | 10.6% |
| 11 | Oakland | 58.6% | 48.2% | 10.4% |
| 12 | Chicago Sox | 57.5% | 47.6% | 9.9% |
| 13 | Detroit | 50.4% | 40.9% | 9.5% |
| 14 | St. Louis | 60.3% | 51.0% | 9.3% |
| 15 | Milwaukee | 51.0% | 41.9% | 9.1% |
| 16 | Atlanta | 59.3% | 50.5% | 8.8% |
| 17 | NY Yankees | 64.6% | 55.9% | 8.7% |
| 18 | Arizona | 51.9% | 43.2% | 8.6% |
| 19 | LA Dodgers | 57.1% | 48.6% | 8.5% |
| 20 | San Francisco | 56.3% | 48.1% | 8.2% |
| 21 | NY Mets | 52.9% | 45.7% | 7.1% |
| 22 | Cleveland | 51.9% | 44.9% | 7.0% |
| 23 | Florida | 54.0% | 47.1% | 6.9% |
| 24 | San Diego | 51.6% | 44.9% | 6.7% |
| 25 | Kansas City | 44.2% | 37.6% | 6.6% |
| 26 | Baltimore | 46.2% | 39.9% | 6.4% |
| 27 | Cincinnati | 50.5% | 44.3% | 6.2% |
| 28 | LA Angels | 59.5% | 53.5% | 6.0% |
| 29 | Chicago Cubs | 52.0% | 48.1% | 3.8% |
| 30 | Philadelphia | 56.4% | 52.9% | 3.5% |
'HFA' refers to Home Field Advantage, and I calculated it simply as the difference between the home and road columns. I'm not sure if this is the best way to do it, or if I'd be better off with a ratio or something else, but no matter what you do with the numbers, you get the same result - at least over the past nine seasons, the Mariners have had a home field advantage that's a little higher than the league average.
And at the end of the day, that's what we want. We don't want the Mariners to be able to score more runs at home. We don't want the Mariners to be able to allow fewer runs at home. We want the Mariners to be able to win more games at home. And Safeco, for the better part of the last decade, has been kinder to its chief residents than most other stadiums in the league.
Why? As a general rule of thumb, the quirkier the home ballpark, the bigger the advantage. It isn't a huge advantage, by any means, but when you think about it, it makes intuitive sense. A team can be built for a quirky ballpark. A team can grow accustomed to the quirks. The Red Sox, for example, will tend to be a little better at handling balls off the wall. The Rays will tend to be a little better at seeing balls against the dome roof. The Mariners' players will tend to know how to handle the field, and the Mariners' front office will tend to know how to build a roster to take advantage of the dimensions.
The magnitude of a team's home field advantage isn't entirely tied up in the way its stadium is built, but it's probably a big factor, if not the biggest one. And the more quirky a stadium is, the more fit it is to be taken advantage of by the host. Or, on the opposite end, the more cookie-cutter a stadium is, the less overall difference there will be between the home team and the visitor. The less the visitor has to adapt, the better its chances of winning.
Don't make more of this than you need to. The Mariners' home field advantage isn't enormous, relative to the others, and it never will be. But to some small degree, we should expect it to be real, and when we look at the numbers, I think we see it. Safeco is a unique stadium for which the Mariners have and will continue to build their roster, and that - along with the player familiarity with its little idiosyncrasies - seems to give the team a bit of an extra edge that I have no interest in seeing potentially reduced or destroyed.