I think this has some good info on paddling technique - dude was a former Olympic flat water racer:
http://www.micheleramazza.com/en/technique/129-how-to-paddle-kayak-technique

Here is some good footage of Marco who uses what I would consider a very wide grip:
https://www.youtube.com/watch?v=QnCY_RQHdpI

Here is video of another buddy who uses a very narrow grip:
https://vimeo.com/132134819

These guys are both great paddlers, using very different techniques.

If you think of your paddle as a lever, the longer the lever, the more power you can exert on your blades. One of the biggest downsides of of the narrow grip is your lose accuracy with your blades.

To demonstrate this, sit on the ground with your paddle, and pick a point on the ground you want to touch with your paddle blade. Try to touch that exact point using both a very narrow and very wide grip. You will see it's far easier to touch that point with the wide grip.

It's a balancing act between getting that leverage a narrow grip gives you, and the accuracy of the wide grip. As a beginning paddler, you don't really have a lot of technical paddle skills, and often times you don't know where you should be putting your paddle. That narrow grip will work well because it will give you a lot of power. It also allows you to keep your hands more centered in relation to your body. This adds another element of stability that a wider grip takes away.

When you envision the arms at right angles hand position, you will see that your wrists are unbent. Keeping your wrists straight is another advantage of this hand position. You are transferring a lot more of the paddling forces to a much larger and stronger joint. If you watch the paddling technique video I posted earlier, you will see him talking a lot about keeping your wrists straight. I would say that wrist pain is nearly as common as shoulder pain among paddlers. Keeping your wrists straight during paddle strokes - see ~1:48 in that video for a good demonstration - will really help prevent that.

I disagree that braking/reverse sweep strokes are bad. It may be a bad habit to use them in situations where you really need to focus more on speed, but they allow for very fast and tight turning. Personally, I frequently travel slower than the current. Maybe that is never a good idea on class V or more creeks, I dunno, but it's served me pretty well up to class IV so far, and I'm never gonna do anything harder than that.