Cyrus - there's no point asking Polo to explain. If you've read any of his posts recently they're all like this and I'm yet to see him explain anything
That's because people always expect answers without even thinking for themselves.
Anyway, so long as your ally is pretty decent, being public at the start can be advantageous as you won't get random incs. For example, SotB were public from the start and had no random incs. We did get organised attacks from a couple of allies but they're pretty easy to take care of. I seem to remember Hell being public at the start and getting absolutely no incoming although my memory may be failing me.