Not every person getting married now needs everything to start a new house. Many have been on their own for a while and already started setting up their own housekeeping so it's helpful for people to know what they need and what will go with what they already have, and also know if they just aren't the type to need a crystal punch bowl or an egg plate!
The same is true with babies. Most people find out what the gender of their child will be, and the registry is a helpful way of relaying that as well as lifestyle choices such as only using organic cotton clothes, whether the mom is nursing, or if they will use disposable diapers. It's also helpful to indicate (by omission) what you DON'T need if you are borrowing from someone else, getting hand me downs, or have already purchased something yourself.
Now you can even set up more "specialized" registries. I sort of had one at a small local shop when I graduated high school. It basically consisted of writing what my luggage pattern was, what colors I was doing in my dorm room and where I was going to school on an index card. Whenever I got a gift they would keep track of it so if someone came in they could find that info out and know if I already had the makeup bag but still needed the jewelry case to avoid duplicates. Not everyone used it, but it was a nice option for people to have.
That being said, there are still WRONG ways of doing registries from both a practical and social standpoint. Here are some of my handy guidelines
1) If you register at multiple places, DO NOT put all the same items on both registries! Your average shopper isn't going to compare all your lists before setting out to purchase. They will pull the list for the store they are patronizing. And if you aren't cross-checking your lists, you are going to get duplicates. Avoiding this is the biggest benefit of registering in the first place. Big ticket items are usually my only exception because one place may have it on sale or someone may have a coupon that would encourage them to make that purchase from one store over another.
2) Not every occasion requires a registry. Just because a store has the option of setting up a "birthday" registry or some other random occasion, that doesn't make it a good idea.
3) Shower invitations are the ONLY invites it's ok to put registry info on (and even that is sometimes debated). The whole point is to "shower" the person with gifts. And you don't host these things yourself. Would you ever put a wish list in with the invitations to your birthday party? I certainly hope not! And would you ever look someone in the eye and say "Please come to my party but only if you get me something off this list of things I want"? If you would, I'm not sure I want to be your friend.
4) Just because you want something, that doesn't mean it has to make the list. Yes, you should put stuff from every price point on your list. But do you really need to put the sink stopper or the extra light bulbs or the trash bags on your list? A good rule of thumb - if it can be picked up during your regular grocery trip, don't register for it! The only exception MIGHT be diapers, and I'm still somewhat torn about this one.
5) Check your list once you get it set up. When you are doing in-store registries, it's almost impossible to delete something once you have scanned it. And it's certainly possible to change your mind in the process or discover something later you like better than an earlier selection. But once everything is said and done, use any on-line editing feature available to you to delete the duplicates, check your quantities, and make sure only your final selections are on the list. If you have 3 high chairs and 4 bouncy seats or 2 different sets of pots and pans and 6 sets of sheets, that's really going to lessen the chances of your getting the one you actually want!
6) Ongoing lists are nice, as long as you remember to maintain them! If you throw stuff on your list, don't get it, and a year or two later your tastes change, take it off. Also make sure you keep up with your own purchases to avoid duplication. If you're going to keep something like this and make it available to others, try to update it at least once every 6 months. If you have more time and energy, before and after major holidays or getting a bunch of gifts is best!
7) Just because it's on the list, that doesn't mean you'll get it! And just because you have a list doesn't mean people will use it.
8) No matter what you get, say thank you and mean it! Appreciate the thought that went into any gift, card, or presence of the individual.