The reduced upgrade time will likely making a larger percentage of their customers eligible to upgrade to the iPhone when it arrives. Rumors have suggested that Sprint may offer the iPhone 5 at launch, although conflicting reports suggest that they may only offer the iPhone 4 and gain the iPad 2 on their network.
Sprint suggests that these changes are necessary in order to keep offering unlimited data plans, network upgrades, and access to the increasing numbers of 4G-capable devices on their network.