But is very usefull when one have to make versions for many platforms: using Universal Apps paradigma answer the problem for Windows Store 8.1 / Windows Phone 8.1, but when the scope must be more wide?. PCL don't resolve all issues and have to rewritte a lot of code of the presentation layer, but bussiness and data layer of the App can be shared using PCL.
My suggestion is to use "shared projects" instead for sharing your source code among multiple platforms. It's like having a PCL project, except you don't compile a PCL binary, but just compile the source straight into the app.
That's not correct. You can use shared projects for ANY project type (WPF, Universal, Console, Silverlight etc), and both VB, C# and C++. In fact almost all the code in the .NET SDK are in shared projects, and then compiled for each platform, with only a little bit of platform specific code here and there.
Also this extension might be helpful for you if you're using VS2013 (2015 has this built in):
Before this extension existed, you had to do it manually, which I blogged about here: SharpGIS | Behind the scenes of “Universal Apps” That should give you a little more understanding of how shared projects really work.
Thanks, in fact, I am developing a cross-platform app with Xamarin.Forms (iOS, Android, UWP) and ArcGIS Runtime SDK for .NET using "shared projects".