It can be quite embarrassing when you're joining a meeting and don't notice that you're not muted.
This happens quite frequently when someone else joins a bigger meeting. You then join the meeting as well by simply confirming the popup and - if there's still time - you continue working and don't check your settings.
Then you may say something to your colleagues in the same room or talk to yourself while working.
Therefore, I'm sure that most Teams users would really appreciate such a setting (we had it in Skype).
Thanks very much!