My site uses LOTS of forms and almost all of them use similar parts. To expedite maintenance, I create single instances for parts that frequently reoccur and then include them using SSI, etc..
For example, my forms have a field called "How did you hear about us?" It's a long list of options in a dropdown menu. I hate the thought of having to recreate this option for every form and then, having to update all the forms later if I need to make a change.
It would be great if admin could create a custom field instance, save it (say under new sidebar section called 'custom fields'). That way, next time I need to use that custom field I can A) add it with one click and B) if I make any changes to the custom field, it would globally update all forms using that field.