Favourite vs field is a very clever idea of Reads. When they offer odds for the field they get to lay horses noone would normally back.
If you don't like the fav. and want to bet against it you would not normally place different size bets on every other runner in the field to beat it.
That is what you are effectively doing when you take field odds against the fav.
However, that does not mean the bet is no value. With truly false favourites field odds are often very good value.