i've made this modification by hand to my JIRA installs and it is very useful. If you have submit buttons above and below every form, it prevents having to scroll after making your change.

This is most useful on long forms, and not particularly necessary on forms that are a single question – like a confirmation. I've done it in such a way that two sets of buttons show up on every form, but it would not be necessary for you to implement it that way.