The problem with quinella's running doubles and the like is that the returns (at least on SUPERTAB) are an unknown quantity. You really don't know what you're going to get - overs or very much unders. I investigated dutch betting on exotics a while back but was beaten by the unknown returns.

Some were extraordinary while others were more than disappointing.
Guess it's a matter of trial and error on races with DECENT exotic pools.