Mainly we're writing because we've just completed a site that lets you upload holidays into Office 365 without the cmdlet.
Note that you can also remove inserted holidays automatically but leaving all other calendar data intact.
The web-based version allows you to do select a list of SMTP addresses to insert at once. In the unlicensed version we limit this to 25 users per day, and only allow you to select US or UK holidays that we have already defined.
The full cmdlet allows you a lot more functionality: multiple time zone support, script-ability, and LDAP query insertions.
For the record, US Federal Holidays came from http://www.opm.gov/operating_status_schedules/fedhol/2013.asp Our default file will insert JUST THIS LIST (i.e., NO “Day after Thanksgiving”) and we left Veteran’s Day a ”Free” day rather than a Busy day,