In suspend, the hard drive is powered down and the contents are still kept in RAM. That's how you can get from a near powered off state to a usable state so quickly. In terms of protecting the hard drive, suspending/sleeping is the same as shutting down.
I've rarely shut down my Powerbook. In fact, I don't remember the last time I shut it down completely. If Giaguara were here, she'd tell you the same thing . I think most Powerbook users don't shut down but use suspend instead.
Don't know what the drain is like. Leaving it on suspend for a day results in about 4% battery usage. I've left it on suspend for a week and when I returned, there was still about 60% charge left. You'll probably use more battery power starting up the laptop than letting it sleep.