When is it a good thing for a hitter to swing and miss? Depending on which baseball fan or coach you ask, the answer you receive may be "never," but that's not entirely true. It's a great thing to swing through a pitch when doing so puts you in better position to succeed than making contact would have -- and there's evidence hitters are getting a bit wiser with plate discipline.
Let's call it the "live to fight another day" theory, and hitters are taking notice.
"Some pitches I do want to swing and miss on," said Blue Jays veteran Chris Coghlan, then with the Cubs, when he appeared on a January 2016 edition of the Statcast™ podcast. "When I'm fooled, I want to swing and miss. I don't want to tap out and be [happy] that I put the ball in play and didn't strike out."
That's exactly right. It's an unavoidable fact of present-day baseball that strikeouts keep going up and up; if 2017's rate of 21.5 percent of strikeouts per plate appearance maintains throughout the year, it would represent the 12th straight year that strikeouts either held steady or increased. To many, that's an enormous problem; to some, that represents the decline of the modern hitter as compared to "the way things were."
But what that feeling misses is that there's no awards given for expanding your zone to reach out and weakly hit a lousy pitch, which often leads to poor contact and little success; the days of high-contact/low-power hitters like Juan Pierre being celebrated for such outcomes seem largely over. If a hitter has the count in his favor, one who goes after a non-strike is often better whiffing at it entirely, allowing the plate appearance to continue, than they are to make weak contact. A "productive out," however heroic it may be in some circles, is still an out.

It's that idea of not making poor contact on bad pitches -- of not helping the pitcher -- that makes the difference between in-zone and outside-zone contact so stark. (We're using the new detailed Statcast™ zones, including the fringes of the plate as "in-zone," and believe it or not, more than 23,000 pitches were contacted beyond the outer edges of the zone last year.)
MLB contact inside Detailed Zone in 2016-17
.275 average
.458 SLG
88.3 MPH exit velocity
98.3 percent of home runs
MLB contact outside Detailed Zone in 2016-17
.116 average
.149 SLG
77.3 MPH exit velocity
1.7 percent of home runs
It's not one-size-fits-all (just look at Vladimir Guerrero years ago, and Corey Dickerson to some extent today), but the gap there is huge. With that knowledge, you'll notice something interesting if you look at baseball's trends over the past several seasons.
Hitters, unsurprisingly, have been making less contact on non-strikes, because they've been making less contact on all pitches. But after years of going after more and more outside-zone pitches in the first place, they've begun to show some more discipline.

You'll notice that in recent years, swings out of the zone peaked in 2015 and have fallen since. As you surely know, baseball's recent home run surge began in mid-2015 as well. While work continues to understand why exactly home runs have increased -- theories, with varying amounts of validity, range from players trying to hit home runs to changes in the ball to hitters hunting fastballs -- it's hard not to notice that fewer swings that are unlikely to lead to home runs follow the same timeline.
What's also interesting is when you compare the percent of swings outside the zone to the percent of swings inside the zone. They generally mirror one another closely, as you'd expect, but as outside-zone swings continue to fall, inside-zone swings have increased this year. 

As we said above, fewer than two percent of homers in the past two seasons have come on non-strikes, and in the past few seasons, hitters have gone after fewer non-strikes, in addition to the larger lack of contact on those non-strikes. While one percent may not seem like a lot, remember in 2016 the Majors had over 334,000 swings. One percent can be thousands of swings that didn't go to potentially bad contact.
If you need a reminder in action of why missing a bad pitch can be extremely beneficial, take, for example, a May 2 homer by Joey Gallo of the Rangers. Against Houston's Mike Fiers, Gallo swung through a 1-1 curveball in the dirt. In his brief career, Gallo hits .096 with a .094 slugging percentage when he makes contact with non-strikes, as compared to .246 with a .612 slugging inside the zone. So while he's an extreme example, had he made contact with that curve, he'd almost certainly have been out.
Instead, the plate appearance continued, and he deposited the next pitch (a middle-middle hanger) into the left-field seats.

To illustrate this, we went back to 2008 to find the ultimate results, using wOBA, of plate appearances (prior to two strikes) after a hitter took a swing at a ball outside the strike zone, depending on whether they made contact or swung through it. When a hitter has one strike and is down or even in the count, contacting a bad pitch is actually beneficial, given how huge an advantage a pitcher has when he's ahead with two strikes. On 0-0 or 2-1, it doesn't really matter so much. 
But when the hitter is ahead, at 1-0, 2-0, 3-0 or 3-1, the benefits of swinging through a non-strike can be huge. On 2-0, for example, 57 points of wOBA is the difference between Corey Seager (.368) and Eddie Rosario (.311). The 96-point difference on 3-1 turns a hitter from Kristopher Bryant (.394) into Dee Gordon (.299). Swinging and missing on bad pitches, rather than contacting them, can be so, so important.

Hitters, it seems, are learning that lesson. We're seeing more swings at good pitches, fewer at outliers, and it's a lot easier to hit those good pitches hard. (Just look at Aaron Judge, who has cut his swing rate at those outside pitches in half, or Oakland's breakout Yonder Alonso, who has missed on those pitchesmore.) 
You'll find examples who don't align with that notion, because there's no one right way to hit. But there's also no consolation prize for reaching outside the zone to weakly tap a ground ball to second base, simply because you didn't strike out. Lay off the bad pitches if you can, don't worry about swinging through them if you can't, and crush the strikes. Crushed hittable pitches turn into home runs. Poorly-contacted pitcher's pitches don't.