If you're working solely from a chart, you can get away with not hearing the vocals (or lead horns, etc.), since you're following the chart, not the singer.
However, in Pop/Rock situations, as mentioned before, accompanying the singer is your major job. Repetitious rhythm parts are hard to keep track of without the lead voice, among other detriments.
But most importantly, vocalists have been known to miss a cue before. And you can't just drive on past them without making it look like you all suck, so you have to be able to react to changes as they happen. That's why they call it "live". And make sure they can hear you, too. You might be suprised how much they cue off you, even if they don't realize it.
Position yourself so you can not only hear the drums, you can SEE the drums, too. That'll help.
Break a leg and have fun on your gig! We expect a full report...