Crash,

I see your point but we're looking at it from different perspectives.
The point is that if you know the average outcomes and can find an edge, you can then get overlays given what the bookies offer.
That doesn't mean you win on the race, but it means you can win on a series of bets.

As to 30% of favourites - so what?
That means that 70% lose and there is a great deal to play with in regards to filters etc.

The whole concept of value is that the bookie has made his market according to that race and not overall value by betting on certain selections.

Example:

If his book is say 110%, that means he has a 10% edge and if sticks to it and correctly fluctuates his markets according to bets laid, he'll win that much on every race.
What he hasn't factored in, and it doesn't matter to him because is margin is always there, is the strike rate of certain horses given certain conditions overall.

This means I should hypothetically lose 10% POT no matter what I bet on - BUT....through purely statistical filtering can get a 10% POT using TAB prices.