I'm genuinely surprised with how many parents attend every performance of everything! I'm an only child, and I don't see any way my parents could have made it to every performance/game! They both worked full-time and had their own hobbies as well.
Thinking specifically about theater, since that was the original topic, my dad and I were both theater geeks. My mom never wanted to be onstage, but she would help build sets or take notes for the director. We were heavily involved with several local theaters, all of which had hugely ambitious schedules. Sometimes we were all involved in the same show, sometimes different shows at the same theater, and sometimes different shows in different cities. I was also in shows at school, which was a private school an hour and a half from home.
And that's just the theater stuff. That doesn't count music troupe, baseball, soccer, basketball or dance, all of which I did regularly at various points in my life. Nor does it count my dad's status as a union rep or a founding member of the Society of Women Engineers, nor the fact that both of my parents finished college and graduate school during my lifetime, nor everybody's individual hobbies and interests.
That said, we were an extremely close-knit family (still are). Everybody made it a point to go see one or two performances of everybody else's stuff (usually the theater schedules were slightly offset, so one show opened and closed a week before the other). We took flowers and gave silly gifts and made the person involved feel important, but we also had our own lives. I think that's really important to a child's development. I loved my parents fiercely, but didn't need/want them over my shoulder at every activity, and they felt the same. Sometimes a bit of breathing room is a good thing. It certainly made for lively dinner conversation catching each other up on what had happened at that night's performance or game!
I suppose it doesn't hurt to attend everything if you have the time. But make sure you're not totally sacrificing yourself and your own life for your kids. I loved that my parents were well-rounded, whole people. I wanted to strive to emulate them and their successes and happiness. I also think I never took their attendance for granted, because I knew about the schedule-juggling they were doing to be there.
But please, whatever you do, don't become a stage parent! That breeds a ton of resentment throughout the troop, and makes the child extremely uncomfortable!
Do you not work? Are there never conflicting schedules? 
It was just cost prohibitive to go to every performance, so I chose opening night and closing night. And that's just how it's been ever since.
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