Would you call scheme unusable? Of course not, people use it every day. It has 12 'fundamental forms' that cannot be implemented within the language itself without a compiler. Every other library function(except low-level IO ones) is defined in terms of these forms. From the language designers, "we realized that the lambda calculus—a small, simple formalism—could serve as the core of a powerful and expressive programming language."
The meat of the S6RS spec is 55 pages [1]. This proves my point rather than refuting it: there's a lot more to Scheme than lambda – 55 pages of design decisions more.
