I've been using one step my entire brewing career up until I bottled a batch this weekend. Never had an issue with one step, but the thought of the quick contact time and spray application of star san made me pick some up. Boy did it speed up my bottling day!
I used to make 5 gallons of one step,dump in all my lines, etc, fill all the bottles, let them sit, then empty them. About 1 gallon of Star-san and a vinator and I only used about half.
One other benefit I found, my lines for siphoning and bottling used to always get water spots (best case scenario, some grew mold and I had to trash them). Rinsing with star-san kept them clean and they dried nice and clear.
I'm definitely sold on using star-san instead of one step, and might take a look at iodophor, a farm supply store is nearby and if it's cheap, it's worth looking in to.