First of all, display.contentWidth and display.contentHeight are supposed to be read-only varaibles (see: http://docs.coronalabs.com/api/library/display/contentHeight.html). You probably should not be trying to change them. The display.actualContentHeight would seem to handle getting you the height of the content area including the letterBoxed bars and you could use that if needed, but other values like display.contentCenterX and display.contentCenterY are set based on display.contentWidth and display.contentHeight. You in code have to change those values if you want to use them.
While you are an experienced programmer and understand the pitfalls of using globals and know when you can safely use them, that isn’t the case for many of the developers out there. We are trying to discourage usage of global variables and while _W and _H are handy short cuts, it’s a better practice to use the values provided with the display object.
The beauty of Corona SDK is things like are not made of right and wrong ways to do things. You should use what works for you. But with the wide level of skills helping the community solve a problem is an easy manner is a good thing.