Obviously, without proper bankroll management, it's easy to get frustrated especially if you're placing big bets.
That 2% rule sounds good in theory, but it really only makes sense if you have a large enough bankroll to enjoy betting. Otherwise, it can get boring over time if you're only playing with small amounts.
Better boring as you say than to flow with excitement and wreck yourself in the process, right? If you can discipline yourself to limit your gambling amounts to that percentage and in order to increase your amounts, you must increase your income. Sure, it can help you to maintain a more balanced gambling/hustling life.
Just imagine having a $100 bankroll and you're limited to $2 per bet, that doesn't sound fun at all.
I do not even think it is per bet, I think what is being explained here is having $100 income and budgeting $2 for bet until your next $100 income comes by. For me I raise the stake to less than 5% which means I can make it 4% or even less based on my gambling inclination for that period of time. To decrease the chances of messing up, I further divide my budget amount across the time I have to gamble before the next inflow and commit myself to not depositing more than I budgeted for a gambling session and never to deposit more than once.