The blinkin' things aren't accurate, and letting them "fire at will" picking their own targets means that they will mow down lots of your own.

I tend to pick their targets for them.

They do not do "area" fire.

This is no problem in sieges, of course, since you can target any building close to the zone you want pounded and rely on the "scatter effect" to get some hits.

In the field, you simply have to wait for a unit in the right vicinity.