That's great for those with a long-term plan. Of course agencies have their uses for others. When I came to Thailand 3 years ago I had no plan at all. I was enrolled for a TEFL course and that was as far as my plan went. I was hoping to work in Phuket, when that didn't materialise (low pay and high cost of living mainly) I headed for Bangkok. It was mid-semester and I wasn't really looking to work in a school, I fancied teaching adults in language centres. The best job of three (in my opinion at the time) that I got offered was with an agency who offered me four days a week in two different schools and as much evening and weekend work as I wanted. I learnt about what to expect in a school and, yes, I became aware that working for a school was suitable for me and "easier" money (due to more sociable hours and lighter work/prep load).
Unfortunately the downside of working for an agency also became apparent, when it was holiday time I was broke because there weren't enough classes to go around.
When term time came round again I had a shiny new school for five days a week which paid my full-time wage and the agency (for whom I stil worked) gave me corporate and language centre classes. It was all very easy to make a very good monthly wage to be honest. Of course I didn't have medical cover and I wasn't promied any holiday pay, but for me (still with no long-term plan) it was all good and I was very happy.
Eventually the school and agency had a row, I was forced to choose. I could stay with the agency or stay with the school. It was a better package at the school (salary, holiday pay, medical cover), but it was a serious decision affecting the long-term. If I stayed with the school I would lose al of my extra work and would have to start networking etc., cushy corporates would have to be sought out.
It was only the fact that I realy loved the school that kept me there, to be honest. Colleagues of mine that weren't so enchanted left and stuck with the agency (they have all now found schools they like also).
What's my point? For newbie teachers with or without a long-term plan, working for an agency is a good way to find ot what's out there and find out what they like. As we know only too well, there are no guarantees that a school will be a good employer - the contract may look better, but that doesn't mean it will be honoured! Working for an agency may not always "lead to" being taken on by a school in which one has been placed (very often teachers are disappointed in this respect), but it is a good way to find out what is available and to identify what kind of permanent employment one is suited to.
I have now left the school and am currenty between jobs, I have one lined up for next month. Even now, every time an eveing course finishes and I have to look around for new contacts to get the overtime pay, I remember how easy that was when working for an agency. There are some terrible agencies out there (I worked for one of the most notorious), but with a sensible head one can get exactly what one wants from them!