Well, my immediate thought about that is that if a guest isn't close enough to the HC to know their tastes and what they want/like, why are they invited to the wedding in the first place?
I do think it varies by social circle. If one of my cousins invited me to their wedding, it would be difficult for me to pick out a good gift for them with no help (like a registry), because I simply don't know them well enough. But we have this genetic, societal relationship
that expects me to attend the wedding if at all possible; and even if I wasn't able to attend, I'd send a gift anyway. There would be negative familial consequences to me not attending. And probably to them not inviting me!
There could also be situations with a couple, Bob and Betty, where the person getting married is connected to Betty, but in their relationship
Bob is the one who picks out gifts for people, with Betty being no help at all. Could be Bob's never even met the person, but it's Betty's cousin or old college roommate or whatever, so both Bob and Betty are attending the wedding, and it's Bob's task to pick out the gift.
Also, even with my personal friends that I know better, I don't always know them in a "wedding gift way." Like, I could pick out some good novels or DVDs for my friend, but I have no idea what sort of towels or kitchen appliances she would like, let alone what her fiance would be interested in. Or maybe I could rule out a few extremes, which still leaves me with half a dozen possible choices.
But where's the experience of giving/receiving a gift if you tell someone *exactly* what you want and they buy it for you? You may as well just ask for the money and/or say "I want £100 to buy X item", which, thinking about it, I think is why I find it distasteful.
Well, personally I think it's fun to look at a registry, and pick out something the person wants that also coincides with what *I* like, or that makes a connection between us. For example, one person I had worked at a lab with and we used scalpels a lot, so I bought the knife set she'd registered for, and made a comment in the card about the scalpels. I dislike the color brown, think it's boring, so if someone registers for brown towels I'm going to skip over that and keep looking, until I see that they've also registered for a cotton candy maker. SCORE! I love cotton candy. So I will buy that for them, and for a brief instant pretend I am buying it for myself. But it's on their registry, so that means they want it (allegedly).
Sometimes Amy allows herself to deviate from a registry in a limited way. For one wedding where we went in on the gift together, we bought the HC the muffin/cupcake pans they'd registered for, and then threw in a container of fun cupcake papers and a cupcake recipe book. The extra things were small enough that they could easily get rid of them (probably) if they wanted, and it made a "theme" package that we had fun putting together. And the main part of the gift was something they'd registered for.