MMOGs as a rule don't use a whole lot of bandwidth in actual operation. However, the quantity definitely rises in busy areas with lots of players, where there are large numbers of mobs, or on raids, and takes quite a much larger jump if you're using voice as well. Most of the content is already sitting on your hard-drive, after all, and the server just has to let you know where it all is, and where it's all going.
The problem is getting that content onto your hard-drive in the first place. The most efficient method -- and quite frequently for users with capped Internet access, the cheapest -- is the optical disk (CDs and DVDs). Large amounts of data can be installed to the drive in only minutes, whereas for many, the cost of an online download of the same data can be more costly than purchasing a physical disk (depending on plans).
Custom content virtual worlds raise the bar considerably. Most clients for virtual worlds (such as the Second Life viewer) are pretty lightweight. If you're up-to-date on your World of Warcraft or Lord of the Rings Online patches, you'd probably scoff at a mere 40MB to download or update the Second Life viewer.
The problem is that that doesn't actually install any significant content on your hard-drive. All of that comes later, at runtime -- because all of the content is dynamic and created with user-content-creation tools. At runtime, all of that content has to be streamed to your computer, which is capable of remembering only about 1GB of it at any time.
On a capped Internet connection, that gives you a limited number of hours per month, depending on what you do, and what content you are exposed to. It all adds up, and it isn't easy to spot -- though users with caps are used to frequently monitoring their Internet usage for the month. Go too far and you either lose access for the rest of the month, pay exorbitant fees for excess data (hundreds or thousands of dollars in excess usage fees aren't unheard of), or have your bandwidth throttled to an almost unusable trickle. Background updaters that are based on peer-to-peer systems like BitTorrent can consume far more of a user's monthly quota than the actual data downloaded to their systems.
Every year the average usage on these capped connections rises -- the MMOG/Virtual Worlds market is expanding. Regardless of where you stand on virtual worlds and MMOGs, more people are coming to the market, more data is moving and the content patches and updates for MMOGs certainly aren't getting any smaller.
For those few of you on unlimited plans, all that means is that your games and updates take just a little longer to arrive. For everyone else, it may mean not playing your favorite MMOG until quotas are reset for the following month, and then managing other usage for the remainder of that month. It represents quite a grim quandry.
It isn't something you can put off for too long, either, because there will be another update, and yet another. Wait too long, and you may as well suspend your account with that game, because you certainly won't be playing it.
Capped Internet access is a vastly more profitable model than unlimited access schemes. With formerly unlimited markets or countries increasingly switching to or trialling capped Internet access, it takes a fairly reckless board of directors to attempt to stick to providing unlimited access plans when their competitors are all but drowning in their own profits. If nothing else, shareholders start bandying terms like "fiduciary responsibility" around, and the dominos start to fall.
Even if caps in those late-switchers amount to ten or a hundred times the world norms, the amount of data required for these games and virtual environments just keeps growing. Do the caps themselves present a threat to the MMOG and virtual worlds industries, do you think, or will the caps expand to outpace the growth?