I definitely consider my money to be safer in a bank account than on a poker site. Not only is there a risk, the company behind the poker site could go bust and not pay players back. There is also a risk, they might accuse you of breaking their TOS in some fashion and confiscate your funds as "punishment".
Not saying that poker players need to be overly paranoid about this, especially if we stick with the larger and more reputable sites. But claiming, that the poker site is some sort of alternative to a bank, is frankly ridiculous. So I would definitely not keep more money sitting on a poker site, than I need to be bankrolled for the games, I play. And if I play mid or high stakes, I would probably also prefer to either have the bankroll spread over multible sites or keep some of it tugged away in a regular bank account.
Just to give some random example lets say, you have a 10.000$ roll, and you are playing 22-88$ MTTs on PokerStars. Then why do you need to have all 10.000$ sitting in the cashier on PokerStars? You are not going to have anywhere near 10.000$ in play at the same time, and it literally takes seconds to top up your account with your credit card if needed. Withdrawals on the other hand takes at least a day even on PokerStars, and on other sites usually more.