For me, winning isn't just about withdrawing your money from an online casino it's also about having control over the money you use for gambling and over yourself. Sometimes, you might be able to withdraw your winnings, but if you end up depositing it back into another online casino because you lack control, you're not truly winning (Based on my own experience). That's why I believe you're real winner when you can control your gambling habits.
Don't you think that a gambler that has the courage to withdraw his win will likely use some of his wins to buy one or two things before he will think of depositing it back to the casino if he lacks self control and cannot relax and enjoy his win.
I think that it is the best thing that a gambler will do by withdrawing his win instead of leaving it in his casino account because he will always be tempted to gamble and that is why one needs discipline and self control to avoid such temptation.