I'm a heavy user of 3rd party apps on my Treo, so I have as much reason as anyone to want them on my iPhone.
My wife is Chinese and probably 10 or 12 times a day we use the English-Chinese dictionary I downloaded from Pleco.com.
I'm a small plane pilot and I have about 6 different programs that I use for navigation planning, checking airport data, and other information. I use most of those programs frequently.
I have a timer that frequently I use for manufacturing related issues.
I'm trying to eat more healthy so I have a diet/exercise planner.
ALL OF THESE are things that I eventually WANT to have on my iPhone. I really NEED them and use them most every day of my life.
But what I also have is a Treo that just sucks. It crashes all the time. Half the time when it rings I can't answer it because it just cuts off. Sometimes it just crashes in my pocket and I don't find out until it's been two hours and I haven't had a call.
I want all my apps BUT more than that, I want a STABLE PHONE! This is what I believe Apple is trying to preserve.
I'm sure Apple could have opened up the whole thing and just said, "Have at it kids!" But down the road a little I'd be hating my iPhone just as much as I hate my Treo now.