The Get Task Form service is intended to be used as part of a UI, rather than instead of one, and thus doesn't provide all the necessary UI components (such as the Ok, cancel & submission handling logic).
The recommended approach is to use Activiti Explorer. In the latest release (beta 2) I added the ability to directly load a task form into the Explorer UI, providing a direct URL to the task form with all the UI components that you would expect from Explorer.