Most home runs are undoubtedly hit off bad pitches–hanging curveballs, tipped pitches, fastballs meant to be inside but ending up over the plate. How often is a well thrown pitch with good location and movement hit for a home run or even extra bases?
Fairly often. Major League hitters are really strong. Watch someone like Vladimir Guerrero - he can hit anything out. It can’t be quanitified, but it happens a lot. A perfect backdoor slider might get hit out to the opposite field when if it were a fastball six inches outside, the hitter would have had no chance.
I’ve seen Vlad get a single on a pitch that he hit after it bounced. I’ve seen lesser hitters plant very good pitches in the stands. So yeah, it can be done, and frankly it is not *that *uncommon.
Every pitcher and hitter plays a guessing game at every pitch. The hitter may guess that the next pitch will be a fastball low and away, and if it is, he probably hits it well, even if it was well thrown and well located. If it’s not a fastball low and away, the hitter has to react. That’s where good hitters seperate themselves from average hitters.
Barkis is right; if a good hitter knows (or thinks he knows) what pitch is coming he can often crush it, even if the pitch is right where the pitcher wanted it.
Unless you define “good pitch” as necessarily being one that the batter doesn’t expect, which is the best kind to throw.
I’ll agree with the sentiment that, in the majors, a good pitch is one that a batter is not expecting. In that sense, it’s very difficult to hit a good pitch well. A well-placed pitch that starts as something the batter expects but ends differently will be practically impossible to hit, while a poorly placed pitch that does nothing interesting will be easy to hit even if the batter is somewhat surprised by it.
A pitch that does exactly what the catcher and pitcher wanted but the batter expects perfectly can be hit well assuming it’s near to the strike zone. Obviously it’ll be more difficult to hit perfectly placed pitches, but given the skills needed to play at professional levels it’s not too much of a stretch for batters to be able to hit well a pitch that’s right where they expect it.
I’m not sure that’s a sensible definition: the pitcher can’t know what the batter is expecting. He can only try to affect the batter’s expectations. You could argue he’s set up the wrong pitch in that situation. But it’s definitely true that a pitcher can make the pitch he wants in the location he wants and sometimes a batter will guess right and hit a homer.
It’s almost unfair to mention Vlad Guerrero in this discussion. If he can reach it, he can get a hit off it. 
You run into a problem here with how you define a “good pitch” - can it have been a good pitch if it resulted in a home run?
That said, many well-executed (in terms of speed, location, and movement) pitches have been hit for home runs. Typically it is because the hitter was looking specifically for that pitch.
Yes.
It’s tempting to define “good pitch” in terms of the outcome, but that’s not how it’s done (at least, not at the kid-pitch baseball level I coach at). A “good pitch” is indeed one that’s thrown where the pitcher wanted to throw it, as you note.
I have read (in Robert K Adair’s The Physics of Baseball, if I recall correctly) that harder pitches are easier to hit out of the park, since there is more kinetic energy in the bat/ball collision. This leads me to suspect that “good pitches” (defined as ones thrown hard and accurately) are somewhat more likely to be hit out than “bad pitches” (again, assuming that we’re talking about both speed and location, not just leaving a hanging curveball over the plate).
Kirk Gibson has an answer for you…
Joe