One more thing. Let me put this into a concrete context, perhaps that'll transfer the idea a little better.
I go to the Kindle store. I only like science fiction, books on Python. I can tell them so, or I can let them figure it out. But either way, that's what they'll show me for specials and so forth. If I enter my email, they can email me (see, no way for them to know my email otherwise unless I actually buy.) So, this, for me, would be good. I see books I want, and I never see another stupid vampire book again. They, in turn, have a customer who is more likely to buy, because (shock) they're actually showing me things I want.
But when I leave the site, all knowledge of me, goes with me. Now, when I'm visiting, say, KangaroosInFancyDresses.com, that crap does not -- can not -- follow me around.
Now, say someone visits the Kindle store using my URL. I thoughtlessly pasted it into an email to them or something, and off they go. One thing will happen, and another might. First, they get Python and science fiction suggestions for the personalized part of their advertising experience. If they buy from those suggestions, no harm done. But second, they may buy something else, such as a stupid vampire book. Later, I come back, a vampire book is presented to me, I hop to my clickable prefs, am hopefully offered the opportunity to unclick "vampire books" or whatever, and off I go.
Is this so bad? Right now, my SO and I use the same Amazon account. I like, as stated, Python and SF. She likes mysteries and cookbooks. So I see those. All the time. It's not the end of the world. What's missing here is the ability to tell Amazon that I am not her, and for our shopping experiences to be differentiated.
I suspect -- I'm just guessing -- that if the limits of how the site knew what you wanted were set the way I suggest, they'd be a lot more careful to show you what you wanted, because it's one of the only avenues left to better the targeting of their advertising.
Anyway, again, just mulling it over. Maybe it truly sucks as an idea. Your thoughts on how to get out our shared cookie/scripting nightmare are?