We have been on 15 cruises now, always main dining, and denied a private table three times. Every time they have sat us with one other couple - a larger table with more people would be better IMO as you have more chances to find someone you get along with.
First time we sailed with a group and sat with them at a table for eight, so it was a moot point. I didnt even know it was communal dining for everyone until we got there.
Second time we just went with it (we could have moved to late dining but it was only a three night cruise, how bad could it be) and really struggled with our tablemates. We clearly had very different views - they were disturbed we didn't have kids by choice and were on a Disney line (they were sailing without theirs) and we were disturbed they told us about sneaking a gun into
Disneyland when they go. This was years ago... I'm sure with the metal detectors that trick wouldn't work any longer. That experience has 100% turned me off of the idea.
The third time we were initially told by our TA (also on the cruise with us) that our request was a done deal and not to check ahead of time, just show up for dinner so we did. Just to have another couple be led to our table where we immediately like, oh there is no way, they must have the wrong table. Nope, the right table.... They sat, we had a pleasant enough dinner but during it the head waiter came over and explained it just was not possible, then my TA's boss who was also on the cruise came over and whispered to me with them sitting there... I am sure we seemed like complete jerks. By the following night we magically had our own table, but running into them on the ship was awkward as heck... they were fine, it would have been fine, but everyone wanted to 'make it right'. Oh well.... >_<
We've since been on river cruises and expedition cruises, land tours etc where you sit with groups and it's always fine, we did prima notte at Palo this past cruise and were with one other couple and it was fun, but I still prefer to have our own table when sailing. Ahead of our Treasure cruise we were prepared to just eat on the pool deck or at Palo the entire cruise if we didn't get our own. Luckily it was fine.