You can use a flat iron to curl your hair. it takes some practice, but you can do it, and i get a much more attractive "bump" on my ends (if i want it, though i usually dont... cause it makes the hair look shorter, lol) using my flat iron.
I think that the deal with curling irons is that you have to hold it on your hair in the same place for a long time, whereas with a flat iron you keep it moving down your hair the entire time. Even if you use a curling stick, which doesn't focus heat on your ends specifically, you have to hold it to your hair in order to get a curl.
I just think that a flat iron is an overall better tool for straightening hair (after all.... it's a flat surface) both from a logistical and hair health perspective.