While a macro lens is obviously made for macro photography, it can also be used to take decent portrait shots. Every macro lens I have seen on the market is Prime. Prime lenses are incredible for portraits due to their sharpness, overall quality, faster f-stops, etc. But when would you use a macro lens instead of something like a standard 50mm prime?
Macro lenses are your only choice to get up close to any object including a model's eyes, lips, etc. While you may be looking to capture the entire model's face, it's always nice to have options. Macro lenses tend to have larger depths of field (higher f-stop) so you may be able to get the entire model's face in focus anyway. Macro lenses are incredibly sharp making skin texture, eyes, and lips pop off the photo.
On the other hand, Prime Lenses tend to be faster and allow you to step back from your model so you're not in their face with your camera.
In the end, nothing beats a non-macro prime lens for portrait photography, but a macro lens is definitely a good alternative for those who don't want to carry more than one lens during a shoot or for those of you who want to get up close to your model.
Good luck and Happy Shooting!