usually custom fields are properties that can be inherited
atleast its easier to clear/modify all the custom fields in the Maniphest Add task view rather than open up a separate window to copy and paste the necessary fields from the parent task
This would also follow the current logic of inheriting everything from the parent.
Already implemented according to doc:
http://www.phabricator.com/docs/phabricator/article/Maniphest_User_Guide_Adding_Custom_Fields.html
copy: When a user creates a task, the UI gives them an option to "Create Another Similar Task". Some fields from the original task are copied into the new task, while others are not; by default, fields are not copied. If you want this field to be copied, specify true for the copy property.