Can't answer the second question as I'm from a different planet am afraid - but as to the first, just do the maths remembering that agency staff are more expensive - either per hour or to recruit.
If the job warrants the extra spend then it's fine because their cost is covered and you can turn them on like a tap. Not having staff can be just as expensive in lost business so you have to factor that in.
About half my staff came to me through agencies. The other half I pick up in petrol filling stations, supermarkets, gyms etc. when I bump into people who just boil over with passion and enthusiasm. They're the best type. There are some seriously good guys and girls out there and they're worth their weight in gold. When I spot one I tend to offer them a job on the spot because I know they can earn their keep blindfold with their hands tied behind their backs. My managing director was working behind a bar when I first met him.