Apple's App Store is a joke, a walled-in garden. There are no adult-oriented apps. For better or worse, Apple feels that we are all not grown up enough to decide whether or not we want adult content on our phones, or if we even want to expand the functionality of the phone into territory it should already be going into anyway. The paid app store is an extension of this philosophy, to extend your iPhone beyond what Apple initially intended it to be.
3G Extender apps break the ridiculous "10 Mb" limit over the air, Intelliscreen-like apps push extra notifications to your lock-screens, and Blacklisting apps do what AT&T won't for you - stop calls in their tracks. All these features could be built into the regular App Store, but they're not, and the developers worked hard to get these apps into great working order.
Developers who work hard deserve more than just a pat on the back by an anonymous community- they deserve some money. How much money they're making is still up in the air, as jailbreak phones make up a very small percentage of iPhones overall. And whether or not the amount being charged is sustainable is another issue. But it's nice to see this even be attempted.