I too have had every iPhone since its inception and have always upgraded to the next one. I never looked back or questioned it. The new iPhone will always improve the faults of the previous, obvious and not so obvious flaws. It's a cycle of flaws and replacements (new iPhone generations). It's really just a part of life, flaws are what keep it going, they are always trying to make it better.
Now, I upgraded to the 4S from the 4 for several reason:
1. I wanted to reset my upgrade date with AT&T. I didn't want to upgrade eligible for so long without a solid date. I'm the one losing money that way. When the new comes out before the next upgrade date, then I'll worry about it then (but I've always gotten my way -- OT for this thread).
2. Camera. I have several P&S and Canon DSLR with 5 lenses and separate flash. It's a pain to carry all of that, but when you see the picture it takes, it was worth it. The iPhone 4S camera's picture looks like it was taken with that, not perfect but close enough for me to leave my gear at home.
3. Speed. Who doesn't want more speed? I don't if it's just age, by my 4 was slowing down/stuttering the past several months. Now, with the 4S, everything is smooth. Everything glides through the screen as my finger does, there's no lag time at all.
4.Siri. Sure, its not essential or maybe a gimmick but it's there. Use it if you want. It's beneficial only if you do have constant meetings and reminder that you input in your calendar - those do take time and it does it almost instantly. As far as the other options it has, it basically takes the place of your fingers for the most part. Use it, don't use it - it's up to.
5. New antenna. Yes the old iPhone 4 did have a problem. It was more severe for me because I refuse to put a case on phone. Now, the problem is fixed with the 4S. I can grip/hold it during a phone call and it won't drop anymore - I don't have to use a headset anymore!
That's it.