Why should I do it more often than that? Linux boxes have been known to go years without rebooting.
Because it provides for safety and reliability. Just because machines CAN go without rebooting doesn't make it a good idea. And reboots are not to make systems run better, they are to protect against the unability to reboot when things are critical. No amount of "systems can run for years without a reboot" matters because that's not related to why we reboot.