Exclusives of any type are a dangerous business decision within the video game business. The growing mentality that they drive sales is caused by short term thinking that doesn't observe the long term picture. Most exclusives can be broken up into one of three categories.
First we the least harmful of the exclusives, pre-order exclusives. Often times in an effort to draw consumers, retailers will broker deals with publishers for small bonuses to offer consumers for choosing their store as the business to not only invest in for a copy, but to purchase from. GameStop is often the biggest payer for exclusive pre-order DLC. This begins to alienate some people who view the company in a negative light. They want to purchase the game to support the company, but not from someone they don't feel comfortable with. They begin to feel cheated by the developer they want to support. Some publishers and developers will eventually offer all of these bonuses to the consumer for a price. Sucker Punch releases all of their DLC offers at some point during the game life cycle, most apparent with their inFamous series. Another side to the pre-order exclusives are region specific. Ubisoft gives more bonuses to Europe for their Assassin's Creed games. As well, some bonuses never are offered in other countries. This form of exclusivity has the least amount of damage to the faith of the consumers and the profit margin of the companies.
Second we have timed exclusives. Microsoft is the guiltiest of this method. The Elder Scrolls series sees DLC add-ons and on the Xbox 360 they are often released at least 30 days before any other platform. This tactic is used to increase game sales for their system. But in the process it slows profit margins and increases risk of piracy. PC players, who supported the series since its roots become more inclined to wait for Game of the Year editions or pirate the game and/or add-ons because they have enough time to see how it is received by others and do not make impulse purchases. Or they might even have a chance to already create modifications that will do what the DLC does, and perhaps even better. This exclusivity will see more damage to faith and profit margins but it can still balance out through the sale of collections of game and add-ons after profits begin to sink from separate sales.
The biggest risk of exclusivity can create large boons or even destroy a studio. System exclusives will make or break any sort of performance. System exclusives often require the backing of a console developer. Microsoft backs Halo, Sony backs inFamous and God of War, and Nintendo backs many of its own franchises such as Mario and The Legend of Zelda. But big risks taken can alienate whole supportive audiences and drastically cut sales. Bayonetta from Platinum Games was a multi-console release on Xbox 360 and Playstation 3. The supporters of the game have now been given what appears to be a singular option should they want to continue support of the developer and series. Bayonetta 2 is slated as Wii U exclusive and while it keeps some of the original supporters and gains new ones, this move destroys a significant part of their profit margin. This sort of move also makes players no longer want to support a developer as they become unsure of what will happen to any series they begin to adopt. The console developers put a lot of money into development of an exclusive which may ultimately cost them not only large amounts of money but supporters as well.
So the question becomes, "Are exclusives even necessary?" The short answer is a disappointing, "Yes, but only for consoles." Exclusives create competition and the competition is necessary for not only quality games, but innovation within the industry. Though pre-order exclusives and timed exclusives have no business in the industry today. People are going to support who they want to, be it platform or store. So there is no need to risk any loss of profit in those methods.
Third party developers should never be creating system exclusives. Especially with similarities between all of the consoles. Creating ports is time consuming, though development time is another issue. With a little effort the cost can be made up and profit margins increased. The fans are more likely to give their support and developers will run less risk of failure. More games, more money, more jobs and more play.