What are the advantages? They don't cite any, but I can see two:
1. Hobbyist implementors can implement "small scheme," so "large scheme" can be expanded with more difficult-to-implement features without ruining the classic educational experience of implementing Scheme.
2. Technical purists who want to use only their favorite module system, exception system, etc. can build their systems on top of "small scheme," so they won't hold the development of "large scheme" hostage or try to fragment the "large scheme" community.
In theory if you're running on an embedded system you can have Scheme without the libraries--much like embedded C, which was C without the standard libraries, something blessed by the standard. But the ones you cite, especially #2, strike me as the most important.
1. Hobbyist implementors can implement "small scheme," so "large scheme" can be expanded with more difficult-to-implement features without ruining the classic educational experience of implementing Scheme.
2. Technical purists who want to use only their favorite module system, exception system, etc. can build their systems on top of "small scheme," so they won't hold the development of "large scheme" hostage or try to fragment the "large scheme" community.
Any others?