But they will only add their photos to one, and that one will ultimately pull ahead.
They're uploaded as 'private' by default, and it's a configurable option, so you can turn the auto-upload feature off altogether. Actually, maybe I did, because I think it asked me the first time whether or not I want auto-upload on or off by default; but even with it on, I don't feel even remotely unsafe -- the photos are automatically backed up to 'the cloud' and aren't shared unless I go in and explicitly share them.
No matter what you personally may feel about it, I can guarantee we'll be seeing blog posts and news stories about "Google put my private pictures I took for my boyfriend on the internet."
Guaranteed.
Admittedly, that's quite probably most people, but, as I said, it does ask you by default, and it does upload those pictures as private until you explicitly go and share them otherwise.