Oh god its the towers street moaning on here.
Get over it, it's a bloody year. After November 2012 noone will be short changed anyway (and tbf, it's probably worth spending £16 if your pass dies in september).
It stops/limits abuse. It stops people sharing capsules between annual pass holders, and it stops one person buying a capsule and using it forever.
Pretty simple to understand, when your pass expires, your annual pass capsule expires. You have to buy a new capsule and renew annual pass to continue using them both.