Consider this scenario: you browse the web, you go to a website with flash ads and leave the window open. You lock it, put it in your pocket, 4 hours later it is dead.

Or this, more likely scenario, you disable flash to improve performance, you go to iGoogle, which autorefreshes every few minutes. Lock it, put it in your pocket. 6 hours later it is dead.

Suspend to RAM might be a lame workaround, but at least it actually works.