As an example: I wrote a clone of racket's for loops. They use #:when and #:break clauses. Instead of generating them when they were present the break clauses just defaulted to #f and the when clauses to #t, meaning that the break clause of the generated code was just optimized away if the user didn't have any break clauses and the test for the when clauses was optimized to a regular (begin ...).

It simplified the code a lot and the optimizer was a lot faster than having to do it all myself at expansion time. I lazily just generate about 30 lines of code for a simple loop that in the end sometimes even is unrolled to the final reault due to guiles optimizer and partial evaluation.