No, it's not needed to retrieve public calendars.

It's required to create calendar user only to create new calendars or to access to private calendars (invitation required).

Some of the possible uses of calendar are:

• Standard Forex Calendar.
• Send/Receive Trading Signals
• Sell your own trading signals or send to your own calendar to add credibility to your signals.

Nobody can send/modify past events (only add future events).