I've offered to share when we have purchased a bottle. Sometimes people have accepted and other times they haven't. There was one time on a "certain" cruise where we purchased a bottle of Riesling and offered to share. It was a larger table and one of our table mates poured himself a FULL glass, took one drink and loudly proclaimed: "This stuff tastes like sh**." and never drank it. The wine was perfectly fine. His behavior irritated me. Others at the table had no wine because of him so I felt obligated to order a second bottle. The kicker was when his wife helped herself to the second bottle but left his glass sitting full.
When we cruised Asia on Statendam we had a wonderful group of table mates and each night we took turns ordering wine. It was a fourteen day cruise and we all were permanent guests at the Captains table so on half the nights the wine was free.
We use to drink more wine but in the last few years we drink very little and order very little on board. In some specialty restaurants we may order a bottle but I can't remember ordering wine in the main dining room in the last four or five years.
A lot of the "cruise etiquette" like, sharing wine, let your table mates know if you aren't going to be there the next night, call the maitre 'd if you aren't going to be there, make sure you are dressed "just so", provide witty but non-offensive humor and conversation, became tiring and too many rules. That is why we do enjoy open seating dining. Yes, you get the same initial questions of "Where are you from? What do you do?" but after the first couple of minutes you can meet some new and interesting people. We have also met interesting people by the pool or in other venues and love to be able to enjoy dinner with them. It's a nice concept.
The good old days were nice but, like most nostalgia, not as good as we remember.