It's a tradeoff between breadth and features. If targeting a profile with Silverlight supports all the APIs that you want or need, then stick with it. You have no choice to make, go for breadth. Excluding test cost, there's no point limiting yourself or your consumers.
However, when choosing a profile with Silverlight or another platform prevents you from using a particular feature (and that feature hasn't been provided out-of-band, such as via Microsoft BCL Portability Pack) this is where you need to make the hard decision. The support feature table on this page Cross-Platform Development with the Portable Class Library can help you with this decision by showing what features you lose/get by targeting certain platforms.
Looking at the data internally, I can see that "Profile78" (.NET Framework 4.5, Windows 8, Phone 8, Xamarin.Android, Xamarin.iOS) is overwhelming the most popular target (~45% of portable projects target this). This is not surprising. It targets large breadth of modern platforms with a reasonable large feature support. These platforms that this profile targets, also support what we consider our "modern surface area", hence have way more shared API surface vs profiles that target platforms that support our legacy surface area (Silverlight, .NET Framework 4). The difference between these two surface areas are covered in detail here: What is .NET Portable Subset (Legacy)?.