Because you switch off the display when you do so.

I used Cyclemeter for months before I got a Garmin 500, and I must admit there are a lot of things I miss about it: colour display, voice coaching, no need to upload to a website for detailed review of rides. Certainly battery life is a disadvantage, but I've logged 100km rides without it running out. Cyclemeter now supports ANT+ sensors, and with the Wahoo ANT+ bike mount it is a serious contender. Garmin really needs to step up its act on the software and display front because Cyclemeter would trounce it on dedicated hardware.