longer rear derailler cages are useful to take up more chain slack. So when you go from a double to a triple, you probably need to update to a long-cage, because a short-cage rear der likely can't take up enough chain slack for the triple to work.
Going the other way, you can use a long-cage rear derailler with a double-chainring drivetrain; you simply won't be using all of its chainwrap capacity.
However, the D/A long-cage rear derailler is plenty light. For comparison:
Dura-Ace long-cage: 212g
Ultegra short-cage: 209g
Even if you were to update to a Dura-Ace short-cage rear derailler, it comes in at 180g. And the difference between 180g and 212g, my friends, is 7% of a pound.
So, if there's any gnawing to be done, it's the more rational part of your brain against the paranoid part of your brain, for worrying about the "extra weight"