Jules 101, I can see your argument but it is these wide spreads that actually present the opportunites to the trader. Without saying too much, let us assume for simplistic purposes that major data will come out within the hour and the quote on the FTSE to finish up is 1.62 to 1.91; you feel that the news has the potential to move the market by 25 points but you do not know which direction it will be in.
Now what is to stop you from placing simultaneous bets of back at 1.88 and lay at 1.65 (abstract figures) at £10 each? If you are wrong and one side gets taken your maximum loss is £10 however, if your bets both get taken, you walk away with a profit of £2.30 (less Betfair commission). There are many other ways of achieving the same financial result or better but that is one of the basic methods.
By doing this, other backers and layers will be drawn into the market and before you know it, the market will be awash with traders.