I'm not saying or implying that "anything that helps one make good predictions, goes". I really don't think instrumentalism is relevant here; if we take it off the table as an option, there still doesn't seem to be any reason to disprefer a theory that posits "objective probability" to one that posits "electric charge", aside from the overall elegance and explanatory power of the two theories. Which are reasons to incline to believe that a theory is true, I take it, not just to see it as useful.