Probably the main reason why (good) managers will fire sales people who mislead their customers is that, no matter how 'Smooth' the sales person thinks they are, they are far more likely to offend a customer than upsale them. Now, this doesn't mean that the sales people can not talk you into a different product but if your sales staff are not looking out for the customer's best interests it can be obvious to your customer.
Basically, if someone walks into your store looking for an XBox 360 and HD-DVD drive so they can play Madden and have a HD movie player it may be appropriate to mention that the PS3 has a Blu-Ray player built into it and is cheaper than an XBox 360 premium and HD-DVD drive. On the other hand, if someone walks in looking for a XBox 360 to play Halo 3 and Gears of War it is (completely) inappropriate to claim that these games are comming to the PS3 and will be better because of the "Power of the Cell" ...
