Hmmm Cook My Tea ?
I'd be able to save power by taking my tea out and holding it up in front of one of these things. If I could hold 50L of water up there for any length of time and then get it home before it cooled I'd be able to have a bath, a 3 monthly event ;-) !!
On a serious note, I don't see this as a good idea :-
I was in the Scouts. We regularly did night hikes, and occasionally we crossed fields of crops. Fried nobbly knees ? Great progress ! Baden Powel would turn in his grave many times. Long live the Scouts because it taught me lots of practical skills, and much more. Health and safety has no doubt stifled Scout activities to filtering tap water before drinking it. Anything else is prohibited.
Birds are actually a pretty effective pest control in their own right and we should do more to encourage their activities. My guess is the microwaves will kill the birds, but leave the insects unharmed. What about slugs & snails that need heat to move ?
They should duct the hot air from political centres around the country to the crops. Even allowing for cooling, the bullshit laden air from brown in parliament will still need mixing with large amounts of cold night air to avoid crop damage.
Aliens attempting to do some crop circle art may get the hump and zap us with a real death-ray.
Brainiac will have to do a series of 'don't try this in the fields' microwave experiments, which I'd actually enjoy, especially the bit about not doing it in your local fields. Which makes me think that these devices could be of entertainment value too if you had metal dishes and wire wool to hand. Hmmmmmm !