There seems to be confusion about the difference between a mechanical system and form analysis. With the later, personal judgement is involved and with the former it isn't.

Being discretionary with the type of races to bet on is 'how the world works' for anyone serious about making a profit at punting. Welters would be at top end of races to bet on and maidens at the bottom end. Giving all of them an equal chance of making a profit from is the main reason most of the punting world looses.