ghumanaman,
I would not call it a rule - just 'proper usage' depending on the context.
Consider the following two statements:
1. There is no rule such as this, which defines the usage of 'such...as' when using examples to define the set (non-restrictive)
2. There is no such rule as this that defines the usage of 'such...as' when using examples to define the set (restrictive)
Do you see the difference between the two in terms of context? Both are gramatically correct but the context is slightly different.
Following the same logic:
"...baritone singers such as Bing Crosby and Russ Columbo..." would imply all baritone singers contributed to the popularization of "crooning", whereas, "... such baritone singers as Bing Crosby and Russ Columbo..." does not imply the same.
Given the context, I strongly feel that we are only talking about Bing Crosby and Russ Columbo, and not all the 'baritone singers'.
In general, "such...as" can be split when using examples to define the set and "as" or "such as" when the words preceding the examples sufficed to define the set.
I am always eager and happy to go a layer deeper while explaining a concept. Thanks for your query.