DHCP Lease Durations

This may sound like a silly question but why is it recommended that DHCP lease durations are reduced in a Windows Server 2003 environment when mobile devices are used.

I can understand roaming allocations where the lease would be very short but why bother in an office environment even if the mobile units are often disconnected and reconnected from the network.

I have come across several sources now that recommend shorter durations than the default 8 days when mobile devices are used including a publication from Microsoft itself but can find no logical reason why this should be.

Hope someone can help.