My scenario: trying to port a small part of an application created by our company from native code (ObjC for iOS / Java for Android) to C-Sharp. The project will interact with o
You can use PCLs currently across many platforms, but it does require some small hacks to your setup.
These hacks are listed in http://slodge.blogspot.co.uk/2012/12/cross-platform-winrt-monodroid.html
Once you've got those working then the functionality available is quite broad - and it definitely includes things like String.Format
For the situations where the PCL profile is not broad enough, then you can use several techniques for extending them - see http://blogs.msdn.com/b/dsplaisted/archive/2012/08/27/how-to-make-portable-class-libraries-work-for-you.aspx . The technique I generally use is to use MvvmCross Plugins - which are basically PCL interfaces with platform specific implementations. But these plugins are generally at the level of 'make bluetooth work' rather than at the level of String.Format
I do lots of PCL work across WinRT, WP, WPF, MonoTouch and Mono for Android - see http://slodge.blogspot.co.uk/p/mvvmcross-quicklist.html for lots of links to PCL work.
It's true that Xamarin have recommended not using PCLs for a couple of years, but that situation has now changed and the official support for PCLs is under way - see http://slodge.blogspot.co.uk/2013/02/the-future-is-portable.html
From a development perspective - especially from the point of view of using refactoring and testing tools - I don't hesitate to recommend you use PCLs now... especially for operations at the String.Format level. However, each project is unique... so it's not always the right answer.
One important note: right now it's better to not reuse the PCL binary files across to the MonoTouch platform - for now, build your portable libraries against the specific MonoTouch library platform. See http://slodge.blogspot.co.uk/2013/01/almost-portable-binaries.html?m=1