I think the phrase used here quite often is "security through obscurity" when it comes to links. The question is whether people feel comfortable with family photos falling under that principle. They're obviously not meant for public consumption, but the feeling of privacy invasion if a random person stumbled on them is going to vary from person to person. If one was totally comfortable with them being public and has zero reservations about random folks peeking on them, then I'd be surprised if there weren't an even sturdier way to do this publicly (through Flickr or just an open FTP link – but that loses some of the convenience of just an iCloud album for some people).