I disagree, I think that the text box is very useful, and that mods should know the rules enough to know which rule a reported post makes, if it does.
Normal members shouldn't be expected to have extremely intricate knowledge of the rules, and be the forum police. That's why the Staff is here, like you said.
A Text Box doesn't limit you to some pre-set "options".
You have the ability to convey what the issue is with what you are reporting, without being bound by a list of options and asking yourself "do any of these really fit?"
Perhaps a marriage of the 2 ideas: pre-set options AND the ability to make a custom report?