I run Thunderbird portable from a data pen, which is why this metric is important to me (i realize it won't be for the majority of users).
Edit; Okay i know i could buy a bigger data pen, but i also do weekly backups to the cloud and am limited to 7mbps upload, 2g takes 40 minutes, so every bit i can shave off helps. But i want to emphasize i realize it won't be important for the majority of users