On a girl I am of the opinion that heels are good for a night out but the rest of the time she should wear what she is comfortable and happy in so if that is flats so be it. Also in heels I prefer the not too high heels, I think sometimes if a heel is too tall so much so that it is difficult to walk in, it defeats the purpose of creating style.
I usually wear boots, so technically I do have a heel but it's one of the blockish ones that mean it's basically like wearing flats anyway, it just gives me a little extra height :P I just can't wear heels, but after seeing these in your link, I kind of want to:
Walking in heels is tiring because you have to use your muscles very differently in order to have a normal/graceful gait than you do when you are wearing flats. I don't mind wearing heels for short periods, but I don't like to wear them for very long periods.