Something I did long ago to the regular COMMENT was made it so you could only comment out lists, e.g. you couldn't just comment out a WORD!.
If you could, this would happen:
>> eval [comment print "hi"]
== "hi"
That's no good... so you can only use things that would evaluate as single elements.
>> eval [comment print "hi"]
!! PANIC: `comment` does not take `word!` for its `ignored` argument
PARSE has a similar issue, though it could combinate a parser and just not call it:
>> parse "aaa" [3 "a" comment across some "b"]
== "a"
But that's of questionable benefit.
In any case, forming a parser rule that's not going to be run is less efficient than just quoting material, which can be done on rules with illegal content:
>> parse "a" [comment [across some "a" ~illegal~] "a"]
== "a"
So the COMMENT combinator takes its argument literally, and you can't COMMENT a WORD!...