Many photographers use macro lenses for taking portraits.
Canon has a few lenses roughly in that focal length area that are quite good (or special) and can be used for portraits. I'm not sure if it's really true that many photographers use a macro lens instead. Let's take a look and see if there's a possible reason why to do this:
- 85mm 1.2
- 135mm 2.0
- 70-200mm 2.8
Let's say you'd want to buy one of these lenses or the 100mm macro for portraits.
The 85mm is 1.2, but focuses slowly, is heavy and a bit of a beast to master. It's a very special lens that is only useful in the right situations. A zoom lens provides great flexibility and the 70-200mm siblings provide great image quality on top. But with up to 1500g they aren't exactly lightweight. Both lenses are rather expensive (compared to the rest)
Then you have the 135mm which is nice sharp, light and does f2.0 for a lower price.
But it doesn't do macro. See, if you came to the conclusion that a 100mm-ish fixed focal length is what you need and you aren't looking for fancyness in either max aperture or variable focal length then the 135mm is great, but if you are willing to sacrifice one more stop of aperture, you could get a macro lens. I think this is the reason why a lot of people use the macro lens for portraits: it justifies the purchase of the lens.
The average photographer is probably not doing so much macro work that he can justify a dedicated macro lens. Now if that lens doubles as portrait prime, what's not to like? Sure it's a compromise, but it's a darn good one. This thought holds true for most manufacturers I guess: If you are willing to buy a longer fixed focal portraiture lens, it's tempting to buy the one that doubles as a macro lens.
what is the sharpest Nikon macro lens
Unless you want to get really good with the clone stamp tool, avoid utmost sharpness for portraits. Lenses are so sharp these days that you can even buy blur filters (to screw on the lens, not photoshop) to reduce the sharpness. What sounds quite insane can make sense in some situation when you don't want to spend a lot of time pixel peeping at an army of pimples and blemishes.
It's also questionable if people want to see that much detail of their own skin.
TL;DR; Buy a 100mm-ish macro lens of your brand if you are in for a solid performing fixed focal length lens that works for both macros and portraits. Keep in mind that very sharp lenses can require more work (make up, post processing, ...) to get "nice" results.