I do think optimization has a place in MMOs, or indeed in any game. It's simple human nature to try and find the best way to do something. Whether it's researching your route to work, budgeting for groceries, or planning out your vacation, most people try to get the best outcome for the least effort in the shortest amount of time. It's simple evolution -- our ancestors that didn't optimize their hunter/gatherer lifestyle either starved or got eaten by sabretooth tigers. Optimization is a survival trait. So it's hardly surprising that we attempt to optimize in games, and feel rewarded by successfully doing so.
The issue arises when optimization in games is done purely for performance, not fun, and at the behest of others rather than being self-driven. I had a ranger/marksman build in Rift. It was quite effective with easy damage and good survivability with the pets. It was also the vanilla pudding of builds, there just wasn't much to it to make it interesting. Sure, vanilla pudding is better than nothing, but I'd rather have chocolate. Or maybe ice cream. I don't care if it turns out ranger/marksman is the optimal rogue dps build, I'm optimizing my characters for fun, not to top a damage meter. Effectiveness is a concern of course, but as long as I can get the job done and have fun, I don't care if my character's performance is optimal. I like seeing big numbers pop up on the screen as much as the next guy, but I'm not going follow someone else's instructions to eke out an additional 5% dps at the cost of my fun.
I think one of the issues with this topic is that people aren't distinguishing between "optimal" and "effective". Your character can be effective without being optimal, and just because your character isn't using the optimal build doesn't mean they're broken. It should also be mentioned that a skilled player with a sub-optimal build will likely perform better than an unskilled player with an optimal build. A character should be effective enough to contribute to a group, but considering anything less than perfection worthless is pretty myopic. Sadly MMOs seem to have created a gaming culture that doesn't try to help people who are actually ineffective, but instead chides anyone who isn't optimal as if they were useless.
This isn't entirely the players' fault. The requirements of a game's design will drive the degree to which players feel the need to optimize. World of Warcraft in the Wrath era was quite easy, and while high end raiders might have been concerned with optimization it didn't seem like anyone else was. I was never critiqued for my build on any of my characters, despite using builds of my own devising rather than something provided by Elitist Jerks. Cataclysm is a whole new ball game though, and if anecdotal evidence is anything to go by I'm sure I'd be corrected on my build/gear/skills/hygiene at some point if I were to go back and try and do level 85 heroics. Why? Because Cataclysm is much harder, so optimization is more of an issue, and even with the limited talent choices available it's still possible to make sub-optimal builds in WoW. WoW also leaves your character completely exposed to everyone around you, letting them instantly assess your worthiness based on talent/gear/gearscore/achievements/sword girth. In a situation like that being judged (and found wanting) by your fellow players is all but inevitable . . . unless you go find the current optimal build on the web and use that.
Ideally characters would be designed such that optimization could follow many paths, and as long as your build was sensible it would be just as effective as any other build. In reality, creating characters in such a way would require sucking most of the life out of them such that build choices had little meaning. That would keep different builds balanced but not be very interesting. Thus character builds in MMOs will inevitably have choices that based on the numbers lead to the best possible outcome for that character. That's fine, there's nothing wrong with there being a "best" build. The problem is when game design forces you to use that best build just to be effective, or fellow gamers insist on you using that best build in order to play at all (whether it's actually necessary or not).
I like optimizing my characters. I'm a min/maxer at heart from my tabletop gaming days. I used to make the most obscenely overpowered mechs in Battletech. But there needs to be more to optimization than statistics, and optimization needs to come from the player's own desire to improve rather than an external force requiring they meet some sort of statistical requirement. There's nothing wrong with going to Elitist Jerks and getting the best build for your character if that's how you want to play, but there is something wrong with requiring everyone else to go to Elitist Jerks and get the best build for their characters too.
I like how Spinks put it best:
I haven't looked at a single website for advice on build design in Rift, and I have no desire to. I've achieved a level of optimization that makes me happy and I'm done worrying about my build. I'm too busy actually playing the game to worry about whether I'm doing it in the most efficient manner possible.