This idea is pretty straigthforward. It would save a huge amount of time for WF designers for WFs that tend to have a lot of approval steps to have WF variables availables as form fields on the left handside ribbon / have form fields be connected to WF variables.
Having to :
Retrieve the values of the previous task for the subsequent one for every field and
Set each field's default value of the subsequent task in the sequence to the value retrieved from the previous task,
while not being complicated in its principle, it pretty cumbersome, unnecessarily clutters the designer and has to be copy/pasted for every task. It also makes handling send back outcomes time consuming, especially when the design is not definitive and is subject to change. Adding a field to a form goes from a simple thing to do to something taht requires careful checking as to where else that field has to be retrieved, inputed, etc.
Having form fields connected to persistent WF variables throughout the life of an instance would be great.