Thank you for bringing this to our attention! I filed Bug 79558: FR> Add reference to System.Linq to all coded steps by default
to address the issue.
I'm sorry it took us so long to respond to your post - we actually took our time discussing whether it was possible to edit the reference template so as to generate a reference to System.Linq "automatically". This was intended as a workaround while you wait for us to add the feature. However, we've finally concluded that the file is embedded as a resource in our DLL and changing it wouldn't be easy.
Again, thank you for the feedback! Please let us know if you have any other suggestions which you would like to see implemented!
the Telerik team