As a clergy couple, we have been to literally hundreds of wedding receptions, bar/bat mitzvah parties, and similar events. I can tell you from experience that in almost every case, unless it's a very small group, assigning tables works much better, for the reasons people have stated. I know some people don't mind, but as you can see in this string, most do. I've heard hosts talking about how they "didn't want it all stuffy" and how everyone loved it this way -- but I also heard several of their guests sighing and even complaining; they were just too polite to tell their hosts.
I dislike having some people seated at assigned tables and others left to find their own seats. I get it, about reserving tables for close family and elderly, but it's still A list and B list, treating some guests differently than others. Why not just finish the job while you're at it? Don't cost nothin'.
Speaking of which, you will save money on rental and centerpieces, and keep your room less crowded, because, as others have said, you need 25-30% extra seats if you don't assign them.
Most weddings here have tables assigned; rarely exact seats. Place cards (I guess they are really called "escort cards") with the guests' names and a table number are on a table near the entrance. If there was a requested meal, there might be a colored dot on the back. I would probably assign specific seats if I had long tables for 30, as one poster above had, or maybe you could assign it by section: "Table 3A," etc., or just call each section of the table by a different table number even though they are all pushed together. Anyway, give the chart to the coordinator or caterer, but I would give cards to the guests rather than just post a chart so they won't have to stand in a crowd to try to see it or even miss it entirely. It's also easier to make any last-minute changes without messing up your neat chart.
I agree that it is the hosts' duty to arrange congenial tables. But having done this several times, I find it one of the most fun jobs of hosting. It's not that hard, and you won't forget anyone. Yes, caterers have more food on hand and know how to squeeze in extra seats if necessary for surprise guests (and remember there will probably be a few last-minute cancellations in any large group anyway).
I also agree it is the guests' duty to make pleasant conversation with whoever is at their table, and not to complain about their seating. *Somebody* has to sit with Uncle Grumpy, near the kitchen or the speakers, or behind the pillar. When it's your turn, don't take it personally, just help the hosts out by taking it as a compliment that you are the best able to carry it off. Remember that guests do not come in convenient groups of exactly 10, and there are going to have to be some sort of miscellaneous assignments.
If I'm seated at a table where I don't know everyone, I put our escort card on the table turned away from me so people don't have to be embarrassed if they forgot our names.
At our daughter's recent wedding, we all had so much fun arranging the tables, and many guests thanked us for putting them with people they enjoyed. The only complaint came from 2 of my high school aged boy cousins who were seated at a table including some kids younger than they were; kids always hate that anyway, and these two both look older than they are and had been enjoying the attentions of a college-age girl cousin of my husband's -- they didn't want their cover blown!
Sometimes we seat natural groups together, especially if it's like cousins or old friends who live far apart, and sometimes, especially for people who see each other all the time anyway, or where people have already been socializing for three days, we mix them up. This is an excellent example of something you shouldn't try to decide in the abstract. Get the list of the real people out in front of you and see what is going to work best with your real group, theory be damned.
I found it really helpful to take a bit of time and print the names of the guests (individually, not by couple of family, so you can see your numbers easily) widely spaced in columns, then cut them into little slips with one name on each. Much easier to move them around and try different combinations that way than constantly writing and erasing, and you are sure you aren't forgetting anyone (don't drop any!). When you get an arrangement that works, write it down and then start again and try another way, too -- like natural groups first, then mixed up but with similar ages/interests/whatever. Maybe a third way, too. Then you can compare them and fine tune. This works great for us, and it's fun, too! A plus is that it is easier than pencil and paper, white board, or computer for several people to all see and work on together, almost like doing a jigsaw puzzle together.