The `comment' form is rarely used due to the fact that the reader retains the form, and it affects the evaluation of the program. Contrast this with the `#_' reader macro, by which the comment is completely removed before evaluation.
For example, let's say you want to comment out the call to "bar" in:
(defn foo  1) (defn bar  2) (defn baz  3) (+ (foo) (bar) (baz)) ;= 6
The following will return a NullPointerException:
(+ (foo) (comment (bar)) (baz))
However, the #_ reader macro works fine:
(+ (foo) #_(bar) (baz)) ;= 4