To put the 50p a day figure in context, if you borrow more than £350 with Monzo you’re getting a better rate than Lloyds (who are about to start charging 1p for every £7 you borrow) are now offering..
So it might sound like a variable rate is better but in the end, if you’re one of the 7/10 people who has an overdraft of £700 on average you’re a lot better off using Monzo’s.
Whether that encourages the ‘wrong’ behavior is up for debate of course 
the major benefit that I can see with this approach is that users who go into their overdraft on day x of the month know exactly how much they’ll pay at the end of the month, rather than having to predict how much they’ll ultimately end up spending & therefore borrowing, in that month.
In other words, it removes uncertainty & therefore, anxiety which is one main benefits of Monzo already..