It's only not a problem if they get alerted via email when people invite them to events. Unlike Facebook, people don't check their G+ every day, so it's more likely that they won't see that event being posted without explicitly being invited by email. And if that's the way G+ events are being promoted, I might as well just set up an email list.
There are opt-out email alerts, and you can also invite people without G+ profiles to the event. The advantages over an email list can be convenience for the sender, the look of the invitation, and additional features for those who have G+ profiles.
