I think this comes under the category of "If your mother tells you she loves you, get it in writing."  There are certainly cases where a written confirmation for CYA purposes is advisable, like when a divorce is pending and you really shouldn't be sure if either spouse is going to remain your client.  And then there are cases where you have known the clients for 20 years, they have always received hefty refunds, and a phone call or voicemail message confirms that nothing was different in 2021.  

Does the AICPA recommend seeking and then reviewing the authorization letters for free?  Or do they recommend adding it to the billing when the return is done?  Is their position totally uninfluenced by the prospect of additional compensation available for dues-paying members?  I can't say, I'm in no position to examine other people's conscience.