I think it depends. If it’s for your own personal use, in the sense that you need to mark events that you don’t intend to share with anyone, then yes, a calendar is a must.
However, for me, if I need to share calendar dates with other people, Proton and Tuta’s calendars are not useful to me because they share my email address with the recipient, which is something I don’t want.
99% of my email exchanges are with email aliases. Not with my Proton or Tuta addresses. If I share a calendar, it’s not going to expose my alias, which I would be fine with, it exposes my Tuta/Proton address, which I don’t want.
I hope that in the future, Proton and Tuta find a way to hide the address behind a calendar. Recently, a business asked me to share a calendar for the event of our meeting and share it with them. I had to improvise. I created an event with Fossify’s Calendar, downloaded it as a file, and sent the file to the business as an attachment. All to avoid sharing my Proton or Tuta address. They didn’t say anything so it was fine. But in the future, I could see this becoming a problem.