Any good experienced twister knows their worth but agencies like these prey on people. They pay on large company's greed to save a few pounds and they prey on twisters desperation for work / income. I am fortunate enough not to be the main breadwinner in my family and can afford to be somewhat picky about what jobs I will and will not take.
Personally these days I prefer not to work for agents. I find as a general rule (although obviously not always the case) the money is not great, it takes ages after the job to be paid (and I wouldn't accept this off a direct booked client so why should I from them?) and often those booking you for the job have no real understanding of what it is that we do so cannot properly relay our needs to the client.
Then there is also the risk of never getting paid! I have heard of a few agencies going under during this recession and of lots of people that will now never get paid for work they have carried out.
Personally I will continue to either work for myself or those I know and trust at an hourly rate of my choosing and will continue to insist on payment either in advance or on the day. At least then if I break my own rules and get stung it is only myself to blame.
.... and there endth the sermon