Rampant? 4800 issues reported in BC over a 5 year period[1], in a province with over 20,000 community mailboxes. Issues include non-theft things like graffiti and other vandalism. Haven't found more detailed info, would appreciate a link if you have one.
1: http://www.cbc.ca/news/canada/british-columbia/super-mailbox...
The level of audacity it takes to assault and rob a government employee, versus casually walking into someone's driveway and grabbing their mail are not even close.
I have people delivering fliers to my mailbox multiple times a week. It would take no effort for them to grab my mail and stick it in their sack.
But again, I don't think "the ease at which your mail can be stolen" should even be part of this argument, as it's apparently significantly low in most cases.