Whenever I sit in a row of seats such as in an auditorium or a movie theater, I prefer to not sit in the middle of 2 people. This is because of the lack of armrest issue.
I'm sure you have noticed that there is usually only 1 armrest in between each seat. This means that only one person is going to be able to use that armrest. The other person is going to have to be squished in with their arms crossed or in their lap.
If you are lucky to have an end seat, at least you are guaranteed 1 armrest. In situations like this, I recommend favoring that side and giving up the other armrest to the person sitting next to you.
The worst is when you have 2 big people on each side who need that armrest space. Then you are surely going to be without a rest for the duration of the show.
I'm guessing the reason for only having one armrest in between is a matter of space, cost, and manufacturing. It's probably easier and cheaper to make them this way, but at the expense of the patron's comfort.
I think for now on, all row seats should be made with 2 armrests each. Sitting bunched up next to people is uncomfortable enough, the least they can do is give you some space for your arms. I know if given the choice between going to a theater with seats that has 2 arm rests and one that has shared armrests, I would definitely go to the one with 2.