I initially thought along the same lines, but not outdoors.

Depending on the energy consumption, this technology might serve as a nice protection inside buildings. Like areas where you don't want unauthorized personel to go to. For everyone else you could "lower the shield" for the moment of passing-through.

That's a good point. You might be able to have a rotating beam passing over a crop, although if aimed at the crop it might have a negative effect due to the surface water heating aspect. Perhaps there are particular frequencies which might be effective against insects, but leave crops unharmed.