I just encountered a webinar that I wanted to add to my calendar. Fortunately
for me, there was a link: "Add this webcast to your calendar:
said. "Fantastic!", I thought, as I clicked the link. And as I clicked through
the link they wanted to manage my Google Calendar.
Um. No. I don't know you. And you don't even need to manage my calendar to add an event. There's this thing called iCal, it's a format that's been around for a long, long, long time, especially in Internet years. All you have to do is provide a link and I can not only add it to my own calendar but you can actually publish updates to the event! Gasp! All without having to manage my calendar, which seems an awful lot like you want to mine what I'm doing.
So no, addevent.com, you will not get to manage my calendar, you will not pass go, but you and your ilk are more than welcome to go soak your head.