In gambling we don't just win without spending and the spending is what gives us to that ability to carefully follow up how much we spend gambling, this also helps us to regulate how we gambles or how reduce the attention focused on gambling. Gambling is pretty much better when you used a particular amount to gamble and that amount isn't something you projected for some other things but solely for gambling so that when you lose it you wouldn't attach too much value to it or even regret why using such amount to gamble.
How will you follow up how much you have spent in gambling, wouldn't that disturb you if you finally know how much you have spent and let's say didn't win up to 30% of the amount you have spend, lets be realistic, getting to know this might even get one into more trouble, some people that got addicted to gambling today didn't just get into that mess just for fun, many of them got to know how much they have spent so far and foolishly refused to give up thinking that they can receiver some of their money back without consdering if those amount can be recovered or not, i don't know if I misunderstood your point but if I do, I will be needing some clarification to it.
I totally accept the point you made about gambling with amount you set aside for gambling but in addition, that amount you set for gambling should also be an amount that you can't get emotionally stressed with, if you lose them to save yourself the stress of thinking otherwise.