I was originally for it, but I've thought about it quite a bit recently and come to the conclusion that all it did was create two separate communities, and a vast rift in between them.
Let's face it; some 10 man fights are easier than their 25 man counterparts, and vice versa. I've raided both in every single tier on heroic and normal while relevant, and I can safely say that. Don't get me wrong, the amount of balancing required in order to properly balance both to the extent they have been is incredible. Let me just start you off with a small word about of this effect:
"Think about cleaning your room for just a second. For some, that might mean just making your bed, and putting your clothes away. Let's say that means you cleaned your room to 99%. What if you were asked to clean your room to 99.9%? That might entail sorting every single thing in your desk, cleaning all the lint and dust off of every surface, vaccuming, neatly re-arranging your clothes in your closet so they look neat. But what about pushing the envelope further? 99.99% clean might entail going through the carpet with a special brush to brush all the hairs out of it left by pets or yourself."
Think about that just for now. Now, consider that that might only be leaving out 1 thing out of 10,000 things. When we have DPS charts and boss health and thousands and thousands of figures to balance and account for, and new abilities, different specs, weapon procs, stat balance, swing timers, EVERYTHING, everything that possibly might need to be balanced, you have your work cut out for you. Now, think about all that math that needs to be correctly tuned with likely thousands of man-hours needed to do so, and then double it. You have to balance both 10 and 25 player raids. You also have to balance PvP to a degree without drastically altering abilities in a pvp environment.
Now with all that being said, just the fact that both are so well tuned as they are (and you know they are well tuned all things considered), that's an achievement in itself. But why would you want to complicate it to such a degree? Surely, with so many things to balance, at one point or another mistakes can and will be made; it's probability. Over a long enough period of time, the chances of a "Mistake" happening is absolute.
Forgive me for dredging this out in a long post, but I don't think a lot of people who say raiding is horribly imbalanced think as to why it might be, or feel that Blizzard simply doesn't give a shit. It's not that they don't care, I think it's just that it's nearly impossible to achieve the same level of balance in one raid situation than it does in several.
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So, my main point after explaining all that is that I believe 10's and 25's will never be balanced. It's just not mathematically reasonable under such a short time frame as to when raids are released. Starcraft wasn't balanced for several years after Brood War was released to the extent it is today, but they didn't have new units every 7 months, or changes to whole buildings, etc.
I would like to say they could fix this, but I don't think they can. If they chose to say "We are going to take away 10 player raids and revert to a Burning Crusade model", they would most likely hemmorhage players. There are about 4 10 player raids to every 25 player raid group now (ranked), and even sparser of a ratio in non-ranked guilds. If they decided to do a full 10 player raid setting, they would effectively destroy the professional raiding scene, and several thousand guilds which generally have the more experienced players and long-term subscriptions. MOST pro-raiding guilds are 25 player because they've been this way since the beginning of Cataclysm. A lot of them have died already because 10s are much more lucrative to new people. Why would you want the difficulty of finding 25 competent people when it's easy for them to join a 10 right away and begin raiding?
As someone once said, Blizzard is effectively in a lose-lose-lose situation. Keeping raiding in the current model clearly has done little good for Warcraft's health in general, and has destroyed more guilds in a year than have most likely been torn apart in the now 6 years I've been playing.
The reason I've stayed with WoW so long? Well, there are a few. Mostly friends. Lore. The fun of raiding. It's fun, but it's been very dull in Cataclysm. Firelands was too small and generic with hardly any memorable lore, and let's face it; Dragon Soul isn't very climactic. It's not because of LFR at all; it's because the bosses are tired, predictable, and in fact, most of them are just re-used graphics.
The reason I loved a raid like Ulduar was because while you leveled, you learned the faces of the bosses. Each of them had personalities, and even inside the raid they had decent scripting. The story about an old god in itself is exciting; they're the true evil in Azeroth and Titan lore. The graphics were some of the best yet seen in Warcraft just in terms of artistic detail, and each model was unique, refreshing, and was very easy to look at. I still get chills thinking about 0 light Yogg; you felt intimidated when you walked into his room every pull a little bit.
I have no idea why Blizzard is fucking up their raids so hard, but I think it's just an unfortunate series of events. The lore around Deathwing isn't nearly as good as Titan lore. Titan lore is more original than a big dragon that wants to kill everyone. Not only that, but who the heck are the bosses in Dragon Soul? They're not just unmemorable, they're practically unknown with not even new lore to encompass them.
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Why I think MOP could be the saving grace of Cataclysm: New lore. I think with total freedom in art direction, lore, and some better mechanics in-game for story telling, I think they could easily make another Ulduar-level raid when it comes to enjoyment and memorability. They also have equal opportunity to totally mess it up with predictable characters or bland storyline. I think they might do it, that is if they aren't actually trying to cut costs. Dragon Soul just felt SO lazy that it couldn't have been just poorly executed, but we could all be wrong.
TL;DR. I don't think 10/25 being "equal" is doing any good for raiding, and I believe they could be successful if they were separate. I also believe MOP could easily produce exceptional content at the level of Ulduar, but could equally be as generic as Cataclysm was if they actually ARE trying to cut costs.
PS: I think I gained some Geek Experience Points there. Gotta love the long-winded posts about a video game, right?