Insects don't seem to have a trouble clinging to walls, so why not create small unmanned drones that can do the same thing? Imagine a flock of these sneaking into a city, hanging out under eaves and in other inconspicuous places, and maybe even seeing through walls. They could silently eavesdrop, and then when the time was right, sneak away and move onto the next listening post.
Whether these are useful for spying or not, we like the idea of appropriating techniques the insects have been using for millions of years, and adapting them for our own purposes. In this case, let's just hope all the spying done by these bug-like aircraft is on the bad guys, and not by the bad guys.