When displaying an issue at a certain step of a workflow, the user sees all fields that have been given a value. When he clicks on a transition that has an associated screen, he only sees the fields he can edit (resp. he can edit all fields that are in the screen of the transition).

It is often useful to see everything, even if you aren't allowed to change everything: For instance, the users need to see the description of the problem while they write a solution proposal and an estimation of the costs, but they're eventually not allowed to change the description.