I'm constantly looking behind me. I look behind me before crossing every driveway and cross-street to check for possible right-hook situations (left-hook situations in the UK.) I also check before changing lanes.

I get by just fine without a mirror but others don't, so a mirror helps.

For some reason my girlfriend hates looking back, so I've considered getting a barend mirror on her stoker bars, so that I don't have to do all the traffic checks when we're riding the tandem.

A mirror is an absolute necessity for me when riding recumbents.