I think the agencies must be filling a gap in the market that was left open by your traditional sparky. People use agencies for the same reason people buy burgers at macdonalds. It's a combination of convenience, consistency and accountability. They know that the guy they get will be qualified, they know he'll do a half decent job at least and they know that if he doesn't they have recourse through a big company (agency) with a code of conduct sympathetic to the consumer.
No,no ,no ! agencies should only be used for short term temp work, instead they are used by companies on all jobs to drive down wages and deny workers basic rights, a bit like the blacks working in diamond mines in S Africa !
Things you will never hear an electrician say -" ive just landed a job cards in with a JIB co paying the national agreed rates !