Other add-ons get absorbed – not DPS meters
The current MMO king, Blizzard Entertainment, has a long history of taking third-party ideas for addons and incorporating them into the standard interface. Remember Outfitter? That's now World of Warcraft's in-game Equipment Manager. How about Omen Threat Meter? Your percentage of threat is now shown on the target's frame, giving damage-dealing characters ample notice to back off or face pulling aggro from the tank. Even the procs from certain talents and abilities are now announced with big, flashing wings and halos with the default UI, eliminating the need to have Power Auras or other similar add-ons installed.
DPS meters are second only to the much-reviled "Gear Score" add-on in the realm of attempting to label and rank players. Blizzard obviously agreed that players lacking the gear to succeed in certain encounters should wait for upgrades. They incorporated their own method for quickly ascertaining the average item level of a player's gear and limited the dungeons for which players can queue in the LFD tool accordingly.
What they have not managed to do, much like the Gear Score add-on, is determine a player's skill at a single glance. Why? Because, skill cannot be measured as such. DPS meters and gear item levels do not determine whether or not a player is capable of utilizing his or her class to its fullest potential. After all, the measure of DPS success is whether the boss lies dead while the player's character still lives on. We need no third-party bit of code to tell us this, the health bars available since very first live build of the game provide that information quite adequately.
DPS meters encourage bad game play
Even in a standard five-man dungeon, DPS meters create a flawed philosophy among damage-dealing characters. For instance, assume that a party's tank gets hit with a curse during a boss fight. This curse deals periodic damage throughout the rest of the fight, causing the healer to expend more mana keeping the tank alive, and as a result, one of the melee DPS falls dead from unavoidable splash damage.
The first assumption is that no character in the party was able to remove curses, but that's not necessarily the case. Instead, the Mage was too busy watching her cast timers in order to maximize her rotation and the Druid choose not to pop out of cat form and take the hit to his own DPS. This mentality of "Me over We" game play ultimately hurts a party that is willing to sacrifice a DPSing character for the sake of individual DPS meter numbers.
The tunnel-vision inherent in DPS-meter attention probably didn't make much difference for this party's success, but it does have one very important and disastrous side effect. The fallen player received the implied message, in no uncertain terms, that he was expendable and ultimately less important than the egos of the other players. What did he learn? That no one else is going to be looking out for him, and he should therefore spend no time looking out for anyone else but himself. That's the type of mindset that is killing the communities of MMOs.
Not all MMOs allow add-ons–this is why
A quick look at Curse.com, one of the leading websites for finding MMO add-ons, shows add-ons for World of Warcraft, Warhammer Online and Rift, as well as for free-to-play MMOs Age of Conan and Runes of Magic. While Curse also hosts an Aion page, there is no "Add-Ons" tab because developer NCSoft does not release the portion of their game's code necessary for third parties to create add-ons. And while Turbine does allow add-ons for their free-to-play Lord of the Rings: Online, this game is of a different class than other MMOS. LotRO does not incorporate any real Player-vs.-Player (PvP) combat and their community is also more lore-centric than other MMOs. For these and other reasons, LotRO does not seem to suffer from the "e-peen" issues of other online RPGs.
DPS meters are simply another form of inter-player competition. Whereas the purpose of five-man dungeons and 10- and 25-man raids are to band players together against a common enemy, all too often the goals of these individual players are singular and selfish: to top the DPS chart. When out-DPSing the other damage-dealing characters in your own group becomes more important to you than achieving a common goal and defeating an encounter, all players involved lose.