There are a couple reasons:

1. Without swinging the bat you are guaranteed not to make an out. If the pitcher has just thrown three balls, you have a pretty good chance of getting a free base.

2. You are likely to see the same pitch 3-1 that you saw 3-0. If the pitcher manages to get the 3-0 pitch over, look for it again 3-1 and put some good wood on it. If not, again, free pass.

Because a batter is successful only 25-30% of the time, take the opportunity of getting a free base, instead of taking your 30% chance of reaching base through other means. If all else fails and the pitcher throws a strike, you still have two more to work with, and you are likely, like I said, to see the pitch again.

Here is some interesting info I came across:

Batting average by count for all MLB players in 2007:

0-0 = .344
1-0 = .341

2-0 = .351
3-0 = .394

0-1 = .324
1-1 = .327

2-1 = .338
3-1 = .368

0-2 = .166
1-2 = .178

2-2 = .195
3-2 = .233

Although, the batting average for hitters when facing a 3-0 count is .394. It is also a respectable .368 when facing a 3-1 count. By taking 1 more pitch, you have about 15-20% chance of walking (by my estimate), however; even if it is a strike the batting average only drops by .026. Therefore, in the long run it is worthwhile to take that one extra pitch.

"2. You are likely to see the same pitch 3-1 that you saw 3-0. If the pitcher manages to get the 3-0 pitch over, look for it again 3-1 and put some good wood on it. If not, again, free pass."

What if you were given a changeup knee high? Is this still the same case?

"What if you were given a changeup knee high? Is this still the same case?"

-You mean, if 3-0 was a change knee high, and you took, would he throw the change again?
I would say it is still the case. As a pitcher, unless you are a pitching around a hitter, with a 3-0 count you will throw the pitch you are most comfortable with at the time, the one you feel you have most control of, and will throw for a strike. If it's a change-up, well, then it's a change-up. Strike. It's now 3-1. You are in a very similar situation. You are still one pitch from a walk, so you want to throw a strike, and most of the time, a pitcher will go back to the same pitch. Maybe it won't be in the same location, but especially if the pitcher is struggling (which by the 3-0 count means he probably has been at least for the past 4 pitches) it might end up in the same location.

-Or, should you swing at the 3-1 change-up after looking at the 3-0 change-up?
I would advise anyone to look for the change-up knee high 3-1, and get ready to swing. If you have any confidence at the plate, are looking change-up and get it, especially in the same location, it would be best to take your best hack. Unless a knee high change is the only pitch you can't hit, hack away. After seeing it go by once, you know what you are seeing, and have virtually no reason not to have your hitting shoes on for the 3-1 knee high change-up.

Personally, I find I hit best when I react instead of thinking. Every pitch, my favourite approach is look fastball and adjust to the offspeed pitch. Even after a 3-0 change-up I personally would look fastball, but would 100% have the change-up in the back of my mind for the reasons I have discussed. The only reason I would not personally look primarily change-up is because I try not to stray from my approach, but for anyone who likes to look for a certain pitch or have a certain approach for certain counts based on certain pitches and all that, looking change-up is probably the way to go.

Hope either of those was the answer you were looking for.

For one, you may want to coax out as many pitches as you can from the pitcher. By swinging on 3-0, you could be ruining a perfectly good chance of getting on base if you pop out, ground out, or get some sort of out(s). There's no guarantee it'll be right down the middle, either.

"The entire Major leagues in 2008 had a .962 OBP with a 3-0 count, with a 3-1 count, .690."

.962? Really? That high OBP could mainly be because of taking the pitch and walking.

"the team swung at 13 3-0 pitches and got 8 hits"

there's your answer: extremely small sample size. throughout the WHOLE year, they swung 3-0 only 13 times. I guarantee they swung 3-1 at least 10 times more.

I would like to see the OBP, not OPS, for 3-0 vs. 3-1, since that is the argument. I would be very surprised to see a higher OBP for the 2008 Phillies 3-1 than 3-0. I will lay dimes to donuts that over the biggest sample size possible for every and all teams, the 3-0 OBP is higher than the 3-1 OBP. Although it is entirely possible that is not the case, as 3-1 is still a great hitters count.

I'm not going to read the entire post because I feel like it.

What I meant was you are given a fastball down the middle 3-0. You said to expect the same pitch in a 3-1 count, (fastball down the middle) instead you are given a changeup knee high. (looks like a fastball down the middle)

So you swing and miss, and the tone has been changed for the at bat.

"great point, but the average is also higher. and overall they get on base a lot more "

Well, I haven't seen a stat for OBP yet, and that is the stat for getting on base. avg and ops are not. Care to provide the numbers for OBP?

I already did but I will gladly repeat it.
2008 MLB
3-0 Count: .962 OBP
3-1 Count: .690 OBP

Forget about walks hits and everything else. The point is, you get on base more with a 3-0 count, theres no reason to take that pitch.

Ok, I misunderstood.

That is entirely possible, but more often than not, if the 3-0 pitch is fastball, the 3-1 pitch will be fastball as well. If you are given a change-up knee high, well, then you better be a decent hitter.

Also, I'm not sure the tone matters at this point. The pitcher has done a good job of fighting back, give him credit. You get your 2 strike approach on, and get ready to protect the zone, you don't want to go down looking.