I always zero out my drives whenever I buy new ones, or reformat for new or major OS updates and installations. You're right, it maps out bad sectors (even new drives can have bad sectors). As for speeding up, it is only (slightly) faster because it is fresh with zero file fragmentation, but it won't be noticeable and really has nothing to do with zeroing out. OS X does a decent job of keeping drives from fragmenting so that is a moot point.
Zeroing out will also buy you more time if your drive is acting flakey. It's not a longterm solution, but it might allow you to use the drive for temporary storage until you can get a replacement. I had a drive that threw a couple of write errors... so I zeroed it out. It lasted another 6 months until it started acting up again. I eventually replaced it (I was using it as a Photoshop scratch disk, so I didn't worry about data loss).
