I remember my first portrait shoot, outside, during the midday sun. It was a hot, summer's day, there were clear blue skies, and the shots I had in my head I could scarcely hold back my excitement for. However, I had never really had a "shoot" at this point — I'd not long had a camera — and armed with two lenses and a flashgun, I set out.
This shoot went reasonably well, but I learned some valuable lessons. For example, I got acquainted with what HSS stood for and when it was necessary; 1/200 sync speed is a long way from sufficient if you want to use a wide aperture during the middle of the day. I also learned that despite the sun being a bright light source, I did not want it as my key light and I wanted it behind my subject. Furthermore, any way I could diffuse or bounce the light from the sun was preferred and the results were far better.
I don't agree with the advice not to shoot when the sun is at its highest point — though it is more applicable to landscapes and other genres — but it does require more know-how and equipment to get the most out of it.