The power coming from the sun is approximately constant. The power hitting any given spot on the planet is far from constant due to weather, day/night cycles and seasonal shifts. For a given solar installation, there might be a time where the ambient intensity (power per unit area) of the light has the cells biased just-so, converting energy at a near ideal efficiency. Of course then the conditions change and this isn't the case any more. When you run into that scenario, i.e. the sun has moved a bit, it becomes very useful to have some kind of mirror or lens to increase the intensity by collecting more of the light and concentrating it to a smaller area, thus bringing you closer to your ideal operating point.

You're correct that at some point you get undesirable transients, or you just melt your cell. I do that by accident more often than I'd like, by shining infrared lasers at a photo diodes while forgetting that I've left a microscope objective in front. Expensive mistakes.

Now re: the article, if you ask me the really interesting thing here isn't being able to focus light to really small areas per se, but that they've broken the diffraction limit. This means you can form an image of an object that is smaller than a half-wavelength of light. For e.g. red light, that means you can take a picture, an optical picture, of something like, say, a mosfet on a chip. Or a virus. That's quite something.

I'll note however that I could foresee (read: speculate) a scenario where having such a tiny focus, like the solar cell scenario, could invoke a quantum mechanical effect in some special material whereby the light coupling becomes much greater, thus increasing efficiency or something. But that's just science fiction as far as I know.

PS: Sorry if this is patronizing. I know nothing about my audience ;-)