How to run third-party iOS apps in OS X?
The reason you can run programs that you have source code and libraries for is that Xcode cross compiles that code to run on x86 architecture inside an iOS simulator. 

What you are looking for is an iOS emulator and that's something that Apple doesn't provide. There are emulators around, but they all seem to be startup / venture funded efforts at this point. Time will tell if they decide to release the product to paying users or otherwise monetize that work. As of fall 2013, there are no feasible options to do what you ask. 

Realistically, you would need to have source code for those third party apps to be one Xcode build away from simulating those apps on your Mac.

This restriction also applies to the dozens of apps Apple ships as part of iOS that aren't shipped in the Xcode iOS simulator.

The TL;DR summary is you need an iOS device do to most development and testing since:


the simulator isn't an emulator 
the simulator  designed for very specific subset of the development and QA/testing tasks needed to ship and support an app
the simulator won't run arbitrary IPA or apps to which you lack the source code