I see the developer fee as just a barrier to entry to submit apps to the marketing place - kinda just to stop the riff-raff. I suspect it also is linked to the the legal agreement as some type of consideration.
As is, they seem to want you to pay up even to deploy an app you wrote onto your own phone. That's just obnoxious. (Googling around, I can find workarounds, but most seem to require jailbreaks or other strange mummery; see http://stackoverflow.com/questions/246422/how-can-i-deploy-a... for a procedure that's alleged to work without requiring that, but which does require other undocumented mummery with certificates.)
Also, it keeps a lot of people from learning to tinker in ways that could develop into job skills later. While $100 is probably a nuisance fee to most of the people posting here, there are a lot of people in the US (to say nothing of the rest of the world) for whom it blows their hobby budget for the month, if not for the year.
The legal agreement is an interesting thought.
As for a barrier to low-quality apps, it doesn't seem to have worked.
Thanks though, if I want to load programs on my iPhone, I'll do so through Cydia.
I cannot afford a MacBook, nor the 100$ fee to develop.