I don't have any experience developing iOS or Cocoa applications, but I imagine that you need to use at least some of the XCode toolchain to make those things happen.
You need the compilers/toolchain from Xcode, but there are third-party build tools, such as e.g. Buck[1] that combined with an decent editor, let you pretty much avoid the Xcode GUI for a lot of the development cycle.
you said "a lot of the development cycle", as someone who hates Xcode I worry that won't be enough (not making anything requiring compilation on mac at moment, so not particularly worried)
I made an iOS app using vim and Lua (via wax - https://github.com/alibaba/wax). Obviously needed the xcode toolchain for compilation and libraries but always from the CLI / scripts.
Intellij is amazing. Note that jetbrains built resharper to de-suckify Visual Studio. And many of the refactorings that we all take for granted were invented by Jetbrains.
No one is stopping 'in theory', but may be because there's no IDE close enough to Visual Studio for making something close enough to Visual Studio stopping people? ;)
Tried to buy a Visual Studio license for porting our game to Windows Phone last year.
Ended up paying $2K for a version that was able to build WP apps, but not allowed to release them.
VS + VA used to be fantastic, today it's just a piece of ... overpriced crap.
But maybe expecting more from $2K+ software (VS) than from FREE software (xcode) is wrong.
Not sure what you are talking about. Visual Studio Express allows Windows phone development for free. Of course, for releasing they charge money (much lesser than what Apple charges).
There are like nine different versions of Emacs for OS X...