I agree that the 30% chunk taken by Apple is a bit large. For iOS, the huge draw (apart from the obvious "it's the only way") is that your pool of potential customers is enormous. You give up 30% of your revenue in exchange for making your software easily available to millions of people.
I'd say try finding a software publisher that will give you a deal anywhere near that good, but if you're already distributing your software electronically...
The ease of discovery may make it worth it. Sure, I may have an existing online store for my software, but people have to Google for it, trust it enough to enter their credit card data, enter their credit card data, etc.
If a significantly higher number of people are going to find and one-click install my product on the App Store, that makes the cut well worth it.
I'd say try finding a software publisher that will give you a deal anywhere near that good, but if you're already distributing your software electronically...