Imo if someone doesn't play through the stories they are not a true GTA fan.
Don't confuse "enthusiast" with "fan." Fan can be casual, or whatever. It's not a big deal.
I myself always finish GTA games. I can't stand unstructured sandbox game play.
Your are right, but too much of GTA's charm and uniqueness is revolved around the missions. Without the missions I would be bored after 3 hours. Without the missions it is just a playground, no different than Saints Row...
I agree with you mostly, but we all are fans, we just have our own ways of enjoying what we are a fan of. If that made sense.