There are two main advantages of re-using buttons :
1) Keeps file size down
2) Ideal for keeping a flowing / constant theme throughout your site.
Personally i only really use invisible buttons for creating dragable MC's.
Invisible buttons are also ideal for using on text, i.e. if the user wants to enlarge a smaller chunk of text. Invisible buttons inside MC's are good, but it can lead to slightly innacurate HIT areas - if you haven extremely odd shaped MC for example.
Hope this was of some help