I really wonder why, because alignment is one of the core things in lisp code style. Take this trivial snippet:
(let ((foo 1)
(bar 2))
(print foo)
(print bar))
The (print ...) statements should just be indented two spaces, so you don't need a monospaced font for that, but the bindings should be aligned... "(let (" in a proportional font generally won't have the same width as 6 spaces.
