Note: I'm planning to do a CGW-style review (read: no score) of HG/SS once I finish a third run with Chikorita as a starter. But for now...
A few months ago, I went into detail as to why a main-series Pokemon game on a console would never happen. If I went to Las Vegas and was asked to set a line on the possibility, it'd be 30,000-1 (or roughly the odds the St. Louis Rams will win Super Bowl 44). But the odds for a Pokemon MMO? 4,000,000-1 (or the Rams win the next 2 Super Bowls).
Oh, I know, the entire Internet would be lined up to preorder it day 1 (or so I heard) and it may be the only game capable of toppling the WoW juggernaut. But there's a lot of reasons Nintendo's never going to greenlight it...
1) Money, money. Yeah, yeah.
The first question has to be how the hell they're going to support the ongoing maintenance required for a MMO. This requires a lot of money, and Nintendo's certainly got the coin to at least get a game off the ground - but that's money that can be used to pay for new tea tables for Miyamoto to turn over.
In addition to standard development costs for a full game, there's ongoing server maintenance, paying GMs and programmers for patches. These things aren't cheap, and any money going into PokeMMOn is money that's not being spent on R&D or funding new projects.
Nintendo's not going to make it a no monthly fee MMO (think Guild Wars) or a free-to-play/microtransaction MMO (think Club Penguin or a lot of Korean MMOs), so they'd inevitably go to the WoW model of base game + monthly fee in order to turn a profit. And when you consider that Pokemon is a series designed exclusively for kids, Mommy and Daddy won't be happy when Billy and Sarah ask for raises in their allowance to pay the toll for the game they got them for Christmas.
(For a further examination of Pokemon's marketing, grab the September 22 episode of ListenUP.)
2) Wrong place, right time
Realistically, what Nintendo platform is suitable to handle a MMO? And is Nintendo Wi-Fi, where you use the 10 digit friend code (cell phone #) to get the 12 digit friend code, set up to handle a MMO?
In order to handle game updates and character storage, you need a system that has usable storage. That leaves the regular DS flat out, as there's no expandable storage. Sure we have 4 gigabit DS cards (used for The Another World, the Level 5/Studio Ghibli joint) but a MMO is going to need even more than that.
The Wii and the DSi both have SDHC support, so theoretically you have up to 32GB available. Even 8 GB SD cards are cheap now - I got a 8GB microSDHC for $35 at Walmart a few months ago. But that may be too much additional expense for players to bear. The DSi has 256MB internal, and the Wii has 512MB. If you're lucky, you'd have 3/4 of that available and it's no way suitable for an MMO. Nintendo would much prefer to make this as easy as possible, but the fact is the earliest system that could be designed for a MMO would be the next rev of the Wii (with HD-DVD drive).
And even then, there's the hurdle of Nintendo Wi-Fi Connection. They haven't even brought in random matchmaking into the DS games, and the only systems that use friend lists and other "modern online features" are 3rd party efforts. Again, this isn't going to change until the next generation of Nintendo systems (both handheld and console). The DSi doesn't count here.
The easiest way out for Nintendo is to put it on PC, but they haven't developed a PC game (as far as I know) in their company's history and would rather fall on their sword than let a 3rd party company handle it. (I'd give it to Blizzard personally, but it'd be delayed to 2018). And can you name a MMO on ANY console in history that could remotely be considered succesful? The only one I'd even think of being in that ballpark is PSO (no monthly fee style) and that ended up dying for the most part with the Dreamcast.
3) Everything you know is wrong
As mentioned in the console post, the developers have said they prefer players trade in person rather than work online. But people always say that there would be a lot of people who would be willing to play different roles in PokeMMOn.
Really cool articles about dancing in Star Wars: Galaxies from CGW aside, I can't think of any MMO that's known for a player base willing to do anything. In Pokemon you'd likely have a makeup of 55% casuals, 40% hardcore PVPers, and 5% in the other roles (collectors, breeders, contest fiends, et al). Eventually, the PVPers would end up running the show based on sheer time in the game, and the other players would get frustrated and quit when they get called a n00b (or prole, or whatever) for the Nth time.
The other thing, and it's a minor thing but one that worries me, is the effect this would have on Pokemon's canon. There's only a few birds, gerbils, Regis etc to go around in the current world (the ones you catch and the occasional user in the Battle Frontier) - but what happens in a MMO? Is there a 40-man raiding group going to get Lugia? And if so, who's going to end up with it? Do they respawn so that everyone and their mother-in-law has one?
There's simply too many things that have to be considered before a Pokemon MMO would get published, and why bother doing it when you can sell 2 million DS games in a week for a tenth of the effort and five times the profit?