As a c# developer coming from the java world. I have taught myself to be better at reading the code then documentation. The only way to be really sure what is going on is to go to the signature and read the code.
From that stand point having the source for a lot of the .Net framework libraries, would really improve my understanding of what really is going on with the program. Especially in the case of obscure bugs where you're just not sure if the framework classes
are holding up their end of the bargain.
With the incomplete and often buggy nature of the framework libraries, this is not as trivial as it might appear at first glance.