I think the Progress Bar can track variables, but you'd be better off using a box and set width than a progress bar I think.
Agreed. Unless, of course, you happen to need a shiny blue heath meter for your game, then by all means go for it.
Though with a couple feature additions the progress bar could be used more frequently. An option for transparent (or different colored) background, and the ability to change the look of the bar (possibly with the image editor, but even a simple gradient changer would work) and you've got a winner.