It's a brilliant model, and will be hugely successful. The people here who are complaining and making the mistake of thinking most mac users are like you.
They aren't.
If you are the kind of person who reads a website named MacRumors and argues the finer points of digital distribution of software, you are in a group that makes up probably less than 1% of the number of mac users out there. Your level of familiarity and sophistication is light-years ahead of most.
I would guess that probably 75% of mac users have never heard of VersionTracker, MacUpdate, or any such sites and have probably never voluntarily sought out & downloaded as much as a single application on their computer in their entire lives. They may have loaded something if prompted as a result of an error message--"to view this content you need the latest version of flash, click
here to get the latest version--but probably have never loaded anything else on to their computer.
If you don't believe me, just go to your local Apple store and casually walk around and eavesdrop on the conversations between the employees there and people buying computers. Really. Just eavesdrop a bit. And tell me how many of those people are taking their macs home, thinking to themselves "Hmm, I'd really like a program that does [X], I'll to go MacUpdate and do a quick search and see what I can find, download a demo of it, try it out a bit, then go to the developer's website and buy the full copy if I like it."
It ain't happening. Just eavesdrop a bit and you'll see what I mean.
Now if you give them a simple 1-click way to "discover" all of these other apps, decide on something, buy it, and install it? All with 1-click, no need to enter any information, no need to worry about how to install it, nothing, just 1-click? Suddenly even a total n00bie mac user can do that. And many, many will.
Plus, many people in that group (and others as well) might be hesitant to start giving credit card info, complete address information, etc. to some random developer's website. Or even if not wary of doing that, too lazy to do so...I know that often I will buy things through Amazon.com's third party vendors (on Amazon.com) rather than from the vendors themselves because if I buy through Amazon I can just buy with 1-click, no need to deal with the hassle of entering new shipping info, credit card info, etc. especially when many sites first demand that you register with them (yet another login and password to remember, or record).
Basically, the app store allows developers to get sales from a whole set of mac users out there--a massive number--who are otherwise totally "unreachable." To be given that for the "price" of Apple taking a 30% cut? There's not a developer out there who wouldn't jump at that.