I can see the self-assessment angle of having some form of total damage meter being attractive.
BUT
1. You have that option already
You can calculate your theoretical top crit damage personally, you can calculate you normal damage - you then apply that to a 20 hit spectrum and get a good idea of your DPS. From self assessment point of view this is all you need - work on this number in theory and you will improve your actual number in practice. Because for you as an individual player - your feat/enhancement/ability profile is set at a point and only the weapon varies OR your weapon is static and you want to apply different feats/enhancements/abilities. Either way you can do some pretty simple math and figure out if it will improve your character.
What is harder to do is applying haste/alacrity figures since the swing per second number is hard to pin down. BUT that number is less important in reality than it is in theory since you are hardly ever constantly attacking unless its beating on a boss.
2. If it is not for self-assessment -
Then it will be a measuring stick used against others for various reasons - given that you need to understand that it will be a non-number since there are too many variables, too many ways to play the game, too many quests with differing styles of objectives, such that any number you get will be fallacious in comparing one character to another.
Heck just a soloer vs. pug vs. guild running in quests will change you total damage or damage per second. Add in Dungeon scaling, hard, elite, epic and you again change the total damage number or even a % damage number.
I will admit I don't know WOW from a hole in the ground, but my understanding is that it is much more definitive in the roles of the players and more constrained in the build paths and mechanics of the game. Thus a DPS meter is a truer measure of a characters impact on the smoothness of a quest. In DDO that isn't the case and a meter isn't needed.