If you are on the punt to make money as opposed to just have some fun you must be anticipating that you will come out in front in the long term. Since none of us can see forward in time to predict our future success the only indications we can gain are from looking backwards and seeing what has happened in the past. Profit on turnover has to be the most accurate measure of whether we have made money or not and if so at what rate we have made it. If this POT is measured across sufficient races, and I am talking about going back at least a couple of years, I see it as being a very good guide to how you are going to go in the future (barring changing your betting methods or something). If you are making a POT over a period of years (or can accurately test your system against past races and guage effective POT) you can bet with confidence going into the future and after balancing a few things like your courage, resources and the regularity of your POT you can decide how much to bet.
IMHO POT is what it's all about.

KV