[DavidZ]

So I have found out some more information that probably answers my own question:

First the PIA is included with v7.71. I thought it only came with 7.8.

The reason why Microsoft says that you should not sign code written by another developer is because of compatibility issues. If you plan on using your application at another organisation for example and they have an interop assembly of the same name it will not be compatible. The PIA version takes precedence. This is why it is always advisable to use the manufacturers version. However normally when you build the interop yourself it is because you are doing so to use in your own organisation and do not have plans to share your applications with others.

As for signing the assembly, this is useful if you want to put the assembly in the GAC (Global Assembly Cache). This enables you to share your assembly - i.e. .NET version of Raiser's Edge API between applications. For more info about the GAC see. Demystifying the .NET Global Assembly Cache - The Code Project - .NET

Thanks for Ron Frum for some of the above information.

David