I'm not really clear why I would use this and not one of the small embedded Schemes that have srfi-18 support.

Chibi and S7 are both "drop some C into your project" levels of trivial to get running, and reasonably zippy.

And there's an upgrade path by using the standard: you can move to Chicken, Gambit, Guile, Racket or Chez and keep your code.