The downside on solar cells is, as always, production cost. Still, commercial cells are peaking around 19% efficiency now (Wikipedia). Last I heard, it was 5%.
The thermal acoustic piezo article specifically mentions solar cells. If the two dev paths become tightly coupled, it may affect solar cell design by enabling heat dissipation to be partly offset. IE a very high absorption material which sheds too much energy as heat to be conventionally efficient may become useful.
I'm not sure what's going on wrt microwave power transmission these days. Wikipedia seems to say it is possible and safe, but somewhat impractical (a "large rectenna volume" is required).
Brilliant. What pisses me off about this is that apparently it isn't even that hard, just no one bothered. Gah.