I can agree with GG to a point.
Advertising is not entirely evil and, if used in moderation in the correct spaces, it can be a very effective way of making money. I'll come back to this in a moment.
Having said that, I fully agree that advertising real world businesses in a virtual world is something I am not comfortable with.
When you build a game world, assuming you have done it correctly, you have a living, breathing, cohesive whole with it's own institutions and assumptions. An MMO, or most any other RPG game is a story and, as the game's creator, you want the player to become part of that story. The more immersed the player is, the more they care about progressing in the game and, the more they care about progressing, the more they'll want to play the game which is exactly what you want from the creator's point of view.
The presence of real world symbology (advertising, for example) shatters a players sense of immersion in the story in the same way that watching a film on broadcast television is not the same experience as watching the same film in a theater setting.
Now, if you have based your fictional world on the real world (in the style of GTA or CoD or others) then seeing a Coca Cola sign or a familiar set of Golden Arches will actually feel 'right' and the player will accept them. The only way for this to work well is if you've built these into the game right from the very beginning. If you try to retrofit them, they will look 'wrong' and it will distract from gameplay. We saw examples of this in our own beloved Paragon City. Ads for Zombcare, and The Clanket, and Pinnacle apartments, and Royal's Books all fit in but the short lived adds for Nike and Burger King just clashed.
There is a way to make this work (told you I'd come back to this.) If your game has a seperate marketplace feature (like PWE's zen store or the Paragon Marketplace) then that is a place where you can allow for as much advertising as you wish. True, this does violate the 'maximum exposure' rule of advertising, but it maintains game integrity. It's bad enough knowing that there is some corporate entitity behing the game just trying to take my money from me (looking at you Sony) without having that fact getting in the way of the story.
As always, my opinion. Make of it what you will.