Cellphones are capable of lasting longer than the window that carriers and manufacturers are providing updates. A two year old smart phone can still make phone calls. By not providing updates to devices, manufacturers and carriers put the consumer at risk and force them to buy new handsets when thier device is still capable of being updated.
By allowing the bootloader to be unlocked, the consumer can attempt to keep thier device secure through groups like XDA, CyanogenMod and others who continue to provide updates for devices when carriers and manufacturers choose not to.