For one, it gives people a sense of control. Sitting in a room, insisting on special treatment, redlining a contract and replacing one set of language with another all make people feel like they're asserting their dominion. And that feels great. But don't imagine that you're going to fundamentally change the provider's contract. I learned this at an SLA presentation given by an attorney. After devoting 90 minutes to the minutiae of contracts, he concluded by saying, "Of course, you won't be able to change the standard contracts much, because they're written to reduce the provider's responsibility. What you're discussing is how much of a service credit you're going to receive."
People also obsess about SLAs because they provide a basis for post-outage haggling. Being hard-nosed up front may mean greater compensation later. But keep it in perspective: no matter how much you haggle, you're not going to be fully compensated for the business loss resulting from the outage."