Well, I think that there are certain restaurants/styles of eating where you are expecting to be in a group with people you may not know (a Japanese steakhouse, or many pubs, for example). If you go into these nights expecting this it can be fun to eat socially. It's all about your mindset at places like this. If you don't want a more social dining experience some evening, these are not the places to go.
I do agree that it can be akward to sit too close to a neighboring party at other restaurants. As a city dweller, all the restaurants are pretty much like this, and so it's all about your ability to be polite. If you focus on the people at your table and everyone else does the same, it doesn't bother me. But yes, after being hot and sweaty out in the parks, and just wanting some down time, if only one table out of a row were removed it would make a huge difference to your elbow room!
We have not had to deal with this that much because we keep a relatively unusual eating schedule. We eat breakfast on the go (pop tarts or pick something up), snack in the midday, and then have an early sit-down dinner in the evening before the crowds, and usually end up in a restaurant with a bunch of empty tables, so we have a nice quiet dining experience.