They're salesmen. They seem to have a good idea what the "popular" thing is. I may go in there to buy something like Alan Wake and they'll try to get me to buy Call of Duty. I guess they are beneficial to the casual gamer but for somebody that does their research and knows what they want, they're virtually useless.
The thing is, they have about 5 minutes to figure out what kind of gamer you are, what you might be interested in, and how to shill accesories, subscriptions, memberships, etc. on you in that short amount of time. Even if these guys have that kind of knowledge, odds are that they won't have the kind of personality to charm you into parting with your money and they'll just come off as douchebags.
As long as my salesperson acts like the game I bought is going to be the best game I ever played, offers some sort of intelligent critique, or doesn't insult my intelligence, I'm happy. I'll deal with that person time and time again because shopping doesn't turn into some sort of awkward social experience (ie: the time the guy in Lennox Mall in Atlanta, Ga made me feel like a loser because I wouldn't pre-order GeoW3 for the Beta).
A good salesperson tries to qualify the interests and wants of the person they are selling to and position the proper fit among what they have, in order to get repeated sales and generate loyalty. In other words, they listen to their customers, and position their products correctly in the mind of customers. A partisan in the console wars won't do that. They are on a mission to persuade people to their console of choice, and ignore the reality of the situation. They score if more people buy the console and the games they choose, irregardless of what the customer really wants. Due to the low wages individuals are paid at Game Stop, this type is who they get. In addition to throwing other stuff on.
And yes, there will be sales quotas and so on that stores will run. So, you go into Gamestop and you get the other stuff, like the power up card (whatever it is) and fanboy bias.