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The Seattle Mariners will have had 18 (since no playoffs in 1994) opportunities to win a four-team AL West when the 2012 season concludes. As of now, they've accomplished that feat three times in 17, one below their theoretical expected number. Good job, Mariners, you're even bad compared against abstract mathematical reality. The division will inflate to five teams starting in 2013 and with that so goes the built-in advantage of fewer competitors that the Mariners have enjoyed, but not really seized, for almost two decades.
However, that's coming to an end with the move of the Houston Astros into the division and the expansion of the playoffs to two wild cards. All that really matters of course is how this affects the Mariners. Who cares about anything else?
As I just recently (very recently) explained here on FanGraphs (go there to read though each team's current probability), in 2012, the Mariners will begin the season with a 31.8%* chance at the playoffs. It's been that way since the switch to six divisions which means they should have been expected to see six trips, all things being equal. Instead we saw four. Great job, Mariners, you look even worse now!
*Each AL West team has a 25% chance at an automatic berth and then a 75% (since they can't win both) chance at a wild card shot, which carries a 9% (1/11) probability. 0.25*1 + 0.75*1/11 = 31.8%. The formula comes from Bayes' theorem.
The odds of winning the division crown fall to 20% (1 in 5) starting in 2013, but baseball is adding a second wild card to the mix. For the Mariners (and for all 29 other teams), that means their level-playing-field probability will then be 33%*. Adding the Astros and a second wild card team actually increases the Mariners' (and Rangers' and Athletics' and Angels') odds of meaningful October play by a very slight margin.
*1 in 5 x 1 + 4 in 5 x 2/12
For those disgruntled Astro fans, they might want to heed this post as well. Their club, stuck in the six-team NL Central and 16-team National League currently face the worst level odds in baseball at just 23.1% each season. Their move to the AL helps grant them an increase of almost 10%.