Good idea, this is also a feature on Google Calendar. IME it is not used much unfortunately, and the challenge is balancing the work a user needs to do up-front vs. the benefits. Definitely thinking about creative ways to get this information though!
I can tell you I'm working in a F50 bank and it's heavily used all the time.
If someone ain't marked required, then that person probably ain't coming up to the meeting, and they won't tell you that they couldn't make it because they assume they're not needed.
Similar thing as in To and Cc for emails. No need to reply to emails where one ain't in the To line.
Ah. Social conventions in large organizations. An infinitely complex topic. But you're probably not trying to cater to Fortune 500 so nevermind.
