Yep, I use them (Betfair)
two practical examples, where an exchange is VERY useful and offers an advantage a book can´t provide
1. let´s say you have taken a future bet @+1900 (for example Formula-1 before qualifying) and the line has moved to +700 (after qualifying) - you now think, that the "market" has overreacted (you don´t see value in the +700 and think +1000 is the correct price) - you now haven´t much possibilities without an exchange, if you can´t sell it at a betting market, like WSEX,..., or if there are no Yes/No options available - at a betting exchange you can make an offer "to sell" @+750 and there will probably be takers, because you offer the best price available
2. if there is a prop/bet at a book available, where you can´t play (nationality, being booted,...), that you like a lot, you´ll get it at a slightly lower price at a betting exchange - let´s say Centrebet (no Americans) has an Oscar prop @+500 and "your" best line is +300 - if you make an offer "to buy" at a betting exchange @+450, you will get it probably at this price, because you are offering a Centrebet customer a small arbitrage possibility
