There are many couples who do. I know just as many couples, who don't shower together. When one person is modest, they don't like to shower, with their partner. As a marriage goes on, you shower less together.
It depends. Me and my girl do occasionally, but there's really not enough room for that to be practical. Maybe with a larger shower area, but we've just got a standard tub/shower. And, you've gotta be careful when switching who's under the water. You don't want to put your arm through glass. :)