I'm not sure this still holds true with the advent of betting exchanges. The market is now pretty efficient. Sure, there's still value to be exploited if you're good enough. However, in general the wisdom of the crowd is quite close to the true odds, at least at the short end of the market.
"In terms of the Sobel and Raines model, a lower conventional bias in the exchanges relative to traditional betting markets is consistent with a higher proportion of ‘serious’ bettors on the exchanges than with bookmakers"
Efficiency in Exchange Betting Markets
http://faculty.london.edu/mottaviani/exchange-betting.pdf
"In terms of the Sobel and Raines model, a lower conventional bias in the exchanges relative to traditional betting markets is consistent with a higher proportion of ‘serious’ bettors on the exchanges than with bookmakers"
Efficient Market Hypothesis
http://www.betfairprotrader.co.uk/2010/12/efficient-market-h...
"If you were to take all horses starting a race at 2.76 then you would find their win rate was pretty close to 36%"
Market Efficiency: An Analysis of the Internet Betting Exchange Market
http://studenttheses.cbs.dk/bitstream/handle/10417/2888/fred...