If your goal is a guaranteed exchange rate of one, then that seems to be almost the only way to do it and therefore ideal to reach that goal. Even in this case

If your goal is a guaranteed exchange rate of one, then that seems to be almost the only way to do it and therefore ideal to reach that goal. Even in this case there is still some risk that you or your bank goes bankrupt but if you start investing the money of your users you either make some additional money for you - which your users probably do not care about much - or you lose some of their money. You would need an investment with less risk than holding cash in a bank account to improve the service for your users. And there is probably something to be said about negative interest rates which would obviously cause you some trouble.