Ah, yes. That good 'ol point-of-view ping pong. Most of us are guilty of having played the game at some point in our writing careers, and even experienced writers can still slip-up in those fast, first drafts. Like my kids, I'm more likely to follow a rule if I understand why it exists (my kids love that word), and the reasons have to be ones that I can identify with. I think most of us can identify with environmentally friendly reasons for doing things. And that brings us to three, eco-friendly reasons not to head-hop.
1. Energy conservation: Imagine a reader curled up on the sofa or lying in bed after an exhausting day. They plan on using whatever energy they have left to relax and enjoy a good book. Suddenly, their eyeballs start flicking left and right. That's right. Just like ping-pong balls. Pretty soon the book slams shut and they're rubbing their temples. Okay. I'm exaggerating a little, but mentally, head-hopping can exhaust a reader. It takes extra energy for them to keep up with POV changes mid-scene. That little bit of wasted mental energy can make the difference between them putting the book down or reading to the end. Don't drain your reader's fuel!
When a reader is kept in a single POV for a scene, they're getting the time and chance to really explore that character's mind, emotions, and motivations. They begin to sympathize with that character and possibly even 'become' that character in their own minds. The reader is investing their time and energy in bonding. If you yank them out of that POV prematurely and start head-hopping, they won't form as tight a bond with the main characters. If the reader doesn't care, what reason is left for them to read on or to buy your future books?
2. Drought prevention: If the watering hole dries up, there's no reason to stick around or return. No insults intended. I'm not calling readers animals (although technically we are part of the animal kingdom). It's just an analogy, one in which the watering hole hold's part of the book's micro-suspense...the suspense created by the reader wondering what the hero or heroine is thinking and how they'll act on it.
By sticking with one POV (either the hero or heroine), you force the reader to read on to the next scene in order to find out what the other character is thinking, or how they will react to what happened in the current scene. Think of how TV soap operas rotate scenes. They keep viewers hanging on by ending each scene with a hook, and then make viewers watch several other scenes before returning to the one they're itching for. It's the same concept. Stick to one POV in a scene, end it on a hook, then switch POV for the entire next scene. The reader stays in suspense, turning those pages until they can get into that particular character's head again. With head-hopping, too much is divulged too soon, leading to micro-suspense drought.
3. Clean air: Things look a lot clearer through clean air. Avoiding head-hopping helps to clear the air. It prevents confusion. By sticking to one POV per scene, we're in essence sorting the hero and heroine's thoughts for the reader. This makes for more streamlined, efficient reading. Why should we expect the reader to flip back and forth or read a paragraph twice because their eyes collided mid-ping pong match? Respect the reader's time and effort. They want to relax, not work. When selling a house, agents recommend that the seller do all the grunt work (fresh paint, wood rot repairs, kitchen and bath updates...) because buyers are more likely to stick around with interest if the house is move-in ready. Keep things clean and clear.
In conclusion, I do have to say that I've read stories by well known authors where the POV is smoothly and skillfully switched mid-scene and it worked. But in their defense, they didn't head-hop through the entire scene. They switched POV altogether. The times that I've seen this, where it didn't bother me, were during sex scenes. There's something about the heat of the moment that helps distract from the switch. Perhaps, symbolicallly, the hero and heroine are 'joined as one' so there's not as far to hop. Okay, that was cheesy, but I couldn't resist (grinning). You get the idea. There are always exceptions, but as a rule of thumb:
Go green. One POV per scene.