Say you're a a rogue: this would allow you to see your energy bar near the enemy's neat plate, so you don't need to look elsewhere on screen when deciding what action to take next.

This has been requested before and I decided against it purely because I believe it would be confusing.
Especially after recently adding the option to display the units resource on the nameplate.

I also know that this is achieveable by using weakauras to attach a simple resource bar to the nameplates. So if this is something you really want it is possible, but I don't have any links to any working WAs that do this on hand.