It is not a limitation of ScaleOut Software assemblies. The SOSS assemblies are built to use .NET Framework (2.0 or 4.x), but you cannot add non-Silverlight .NET assemblies to a Silverlight project, since Silverlight uses a special edition of .NET CLR runtime. Silverlight applications run in a sandbox environment and have limited & restricted access to machine resources (like registry, hard drives, etc.). That is why Visual Studio does not allow adding general .NET assemblies as references to a Silverlight project.
