Cruznut, I agree it is rude, but unfortunately it is often hard to avoid.
Sometimes you don't decide until the last minute that you aren't going to dinner. I guess you could call, but except on a top cruise line I doubt the maitre d' would think "oh, I should check the table and make sure no one is sitting alone."
All I can say is that if you know you won't be at dinner some night you should try to tell your tablemates directly as soon as you know. If it late you could visit ther table and just say "hi, I won't be here, but usually you arent there because you dont fell like getting dressrd for dinner.
If you find yourself sitting alone, I think you could just get up and go back to the maitre d' and ask to be seated at a different table. I never thought about it before but it makes sense.
Let me ask you this - as a person who often does NOT go to dinner, do you think I should tell my tablemates that the first night?
Also - as a single I now think you do better with anytime dining. That way you can always request being seated with other people, and they will be different every night.
These days people just dont show up in the dining room as regularly as they used to.