While it is possible to a certain extent to do zooming with the legs when it comes to short primes, it is not always going to be, and in some cases it just isn't practical. Unless you want to back off the bridge and fall into the ravine below, or take a step forward and go tumbling down the Great Canyon.
Both primes and zooms have their places in photography, and for a person like me who is not terribly concerned about a slight loss of image quality, I'd use a zoom. And I'm lazy. ;p
However I will admit that from my (very limited) experience, the range between 55-70mm is not terribly crucial. From 40-70mm, the difference is more stark though.