The problem is not having a bankroll; it is allowing the bankroll to guide the gambler. A student's performance will not improve simply because he has a personal reading time table; rather, what matters is his ability to adhere to the time table. This is the same for gamblers who set a bankroll. The bankroll is not the protection, but rather the discipline to stick to the bankroll.
I gamble passively, so I do not have a bankroll in mind. When I want to gamble, I gamble, and whether I win or lose, I stop gambling until I am led to do so again.