In general, I completely agree that leaving large sums of money in bank accounts (checking or savings) is not anywhere near as savvy as investing them (though I’ll pick a nit and say that “investing in a bank account” is not an accurate way of describing it, since that’s saving rather than an investment ;)).

However, there are cases that it does make sense to leave even a large sum of money in your bank account:

• You’re planning to make a very large purchase and need the money to be available and easy to move (e.g., down-payment on a home).
• You expect to need the money in the few-months to couple-of-years term (in which case a certificate of deposit a.k.a. CD or a money market account would be better yet, but investing would not be a good idea).
• You’re so wealthy that $50k is a minute fraction of your overall financial assets, in which case leaving $50k in a bank account may make perfect sense.
• You haven’t yet figured out what you want to invest in, and are keeping the money in a high-interest account until you decide.