I love his sentiment about the 'holy cows' of permaculture: I have experienced similar, when someone actually bowed their head and spoke to me in hushed, reverent tones, when they told me "Perhaps now you could make a herb spiral."
But as far as swales themselves: I have one at the top of my garden. It is absolutely along the contour, and catches either the run-off from a roof, or I can let overflow from water butts run into it when the storage is full. It is fascinating how it runs 'down' both ways, whichever end the water enters (hence me assuming it must be absolutely level). So the benefit of this, immediately, is that the water doesn't water-log one area. Yes, it might potentially water-log all the entire length, but in fact it hasn't, even the last couple of days when there has been torrential rain with flooding.
But the most useful time is not when there is a large amount of rain, but when there isn't. Unless you have some technical system where you can vary the gradient, speed of flow, etc at different times, you need a system that will cope with different levels of rainfall. Light rain in the summer is also dispersed equally along the swale and seeps into the bed below. It's not so much that I am storing water in soil that doesn't need it; I am irrigating soil that does.
I also have what I call an acequia - what Patrick calls a ditch. In my case its function is to move waste duck water down away from the ducks' bowl area. This flows really fast, until it peters out. It is plain that if it was a continual, or voluminous, quantity, it would simply result in too much water further down. In my case, it would end up on the path outside the front door. On a larger scale, it might leave the plot in question and end up on someone else's land, in an already full river or in someone's front room...etc. I have observed that, in dry weather, it runs too fast to be absorbed by the bed that it would preferably irrigate. As it is on a downward slope, and I do want it carried away from the ducks' puddling area, it can't be a 'swale', but I do need to slow the water down.
My plan is to create a series of 'locks', little gates that will temporarily block the water flow and force it to soak in or spread to the bed it flows past. These could be moved from one place to another, according to what needs watering, and how absorbent the ground is at that particular time. It would also be possible to create smaller gated channels to divert water along a different route - like the Moorish irrigation systems - hence 'acequia'.