(By the way, no, the Mariners' slump isn't the reason why I haven't been writing much lately. As anyone who lives in Southern California can attest to, the last few days have been absolutely miserably hot, and when you don't have air conditioning, that can make it mighty difficult to muster up the motivation to do anything more than sit in front of a fan. My brain is not working at full capacity right now.)
There are few things in baseball that manage to get fans as angry and frustrated as they are after watching a batter make an out on the first pitch. It reeks of overaggressiveness, particularly right after a walk or HBP, and it makes it seem like the batter didn't have a plan. With all the talk these last few years about working the count and getting on base, I think a lot of people would rather the players on their favorite team never swing at the first pitch at all than risk making an out so quickly.
That sounds all well and good - you want to make the pitcher work, right? - but then what're you left with? Three-fifths of the time you're going to find yourself behind 0-1 in the count. It's not the end of the world, but it puts you at a severe disadvantage:
Average Hitter: .267/.334/.420, 0.5 BB/K
Average Hitter After 0-1: .238/.281/.361, 0.2 BB/K
Falling behind 0-1 turns an average hitter into Julio Lugo, Ichiro into Orlando Cabrera, and Albert Pujols into Aramis Ramirez. It puts the pitcher in control of the at bat, and it forces the guy at the plate to go on the defensive.
The key for any hitter, then, is to avoid falling behind as much as possible. And the two ways to do this are either swinging at a hittable first-pitch strike, or taking a ball.
So with this in mind, if the pitcher gives you a first-pitch fastball over the plate, why not swing? Where the average ML hitter has a .301 BABIP, it's .311 when the first pitch is put in play, implying better contact (it's .298 after falling behind 0-1, and .290 after falling behind 0-2. Defensive swings and all that). Presumably, hitters who swing at the first pitch are looking for a fastball dead-red, which makes them more likely to hit a line drive if/when they get it.
If the pitcher on the mound is trying to get ahead with his heater, it's in the batter's best interests to try and put the strikes in play - that's the guy's most hittable pitch, and taking a hack is better than falling behind in the count. He has a better chance of getting a hit in the AB if he swings than if he waits.
Going after the first pitch of an at bat is not, in and of itself, a problem. Pitchers know how important it is to get ahead, and while it may seem counterintuitive at first, following a walk you're probably more likely to get a fastball over the plate, as the pitcher doesn't want to fall behind another batter. The way I figure, the more trouble a guy's having finding the zone from the mound, the more likely you are to get a meatball to start off.
Where first-pitch hacking becomes an issue is when the batter in question doesn't have a good enough eye to discern between a cookie and a tough pitch to hit, or even between a strike and a ball. The only first pitches worth putting in play are fastballs over the plate. You don't go up there looking to hit anything offspeed, so you won't put a good swing on a changeup or curveball, and if the pitch is off the plate then you'd obviously much rather get ahead 1-0. First-pitch fastball strikes. It's a mistake to swing at anything else to start off an at bat.
When Raul Ibanez swings at a 96mph Jeremy Accardo fastball over the plate right after Jose Guillen gets beaned, that's fine, that's the right idea. It was a pitch Ibanez could handle, and he hit it hard, albeit in Aaron Hill's direction. When Guillen chases an 82mph AJ Burnett offering down and inside to start off, though, that's stupid. It gives the advantage to the pitcher, and Guillen didn't see a strike in the entire at bat, eventually grounding to second on an outside fastball. When you fall behind, you have to protect, and protecting the plate means you don't get as good a swing on the ball.
Plate discipline isn't about drawing walks and working the pitcher. Plate discipline is about knowing which pitches to swing at, and which to ignore. It's probably not a coincidence that the Yankees' BABIP goes up six points when they put the first pitch in play, while the Mariners' goes down ten; the Yankees know which pitches they can mash, while too often the Mariners go up there all aggressive-like and try to hit a pitcher's pitch. But more to the point, simply criticizing someone for chasing the first pitch isn't enough. If it was a fastball over the plate, then the hitter made the right choice. It's only worth criticizing a hitter if he chases something offspeed or out of the zone. That's when it becomes as problem. "Aggressive" doesn't always have to be synonymous with "stupid", no matter what the Mariners might make you believe. Done properly, going after the first pitch can be a much smarter strategy than a lot of people would tell you.
(For the record, this is why it's such a big deal for a pitcher to be able to start a guy off with an offspeed strike. That pitcher's going to get a lot of quick outs and 0-1 counts.)