Hookers tend to work at night, so it's not surprising you didn't see many of them.
At the end it is a matter of tastes; I personally find Zona Rosa, as someone else said, trashy and tacky, but for others it might be OK. Each to his own, It's definitely not an area to avoid, but to me it is a place with no appeal. I still go there from time to time to have lunch with a friend who works in the area, but I wouldn't go for a drink at night.
I don't understand the comments about fast food. Zona Rosa has them, sure, but they also have many places that cater to office workers in the area where you can get home-made style with different courses (what we call 'comida corrida'). Plus, places like Los Bisquets or Sanborns, that have good food (not for special occasions, but good for a regular meal).
For sure the area is popular with local residents. There's more than 20 million of us and with all sorts of budgets, tastes and needs. It doesn't have to be sex based either. Many are going to be in the area for all sorts of reasons. You finish work with your friends and grab a quick beer before going home; you might park there if you want to bike on Sundays; you could grab lunch there after visiting one of the many medical offices in the area; I used to wait there for my wife to finish a training she took once close by, etc.