Right now, if a custom form is attached to a task, there is nothing that forces the user to fill out the custom form. The task may be completed, without the custom form from being updated. Even if the custom form has required fields. Only when someone edits the custom form, does it require you to fill in the required fields.
We have examples of collecting information when we get to certain points in a project. This data is being collected via a custom form on a task. However, the system doesn't force the assigned to user to fill in the custom form. They can close the task without even looking at the custom form tab.
As a workaround we have been attaching an approval process to the task, so the PM can verify the information was populated. However, it would be nice to not have to take this extra step.