We need the possibility to create "work orders" for each issue. Your concept of sub-tasks is unfortunately not sufficient, because they don't depend on the requirements. For example the "Fix Version": With this fields a project indicates in which release the issue will be solved. Now when I create a sub-task the "Fix version" won't be the same as the superordinated issue. I have to set it manually. But when I have quite a lot sub-tasks I have a real problem, to postpone an issue to another release. Because in this case the "Fix Version" of the sub-tasks will not be adapted to the issue's value automatecally.
Another problem is, that the remaining efforts of the sub-tasks don't have any influence on the remaining effort of the superordinated issue.
If you don't want to change your sub-task concept, I suggest to introduce the concept of "work order". A "work order" must be strongly bound to the issue where it is assigned to.