Ah ha... that makes a lot of sense!
It was mainly just an annoyance because, for example, a lot of the source solutions relating to Cocktail include "SL" and "SL5" projects along with them, which I would then have to strip the "SL" projects out manually to get the solution to compile.
Out of interest, why is it that the Silverlight 5 assemblies were being created, but not the Silverlight 4, if I only had a WinClient Express licence?