Usually this comment doesn’t mean photographers prefer an unsharp lens. It means the softness may not be a deal-breaker in portrait use.

For portraits, slight wide-open softness can be flattering because it reduces the visibility of skin texture and blemishes. Also, optical “soft focus” isn’t the same as simple blur: the subject can still be in focus, but with lowered contrast from aberrations such as spherical aberration. That look can be hard to reproduce naturally in post-processing, and it often avoids the over-smoothed, “plastic skin” look that heavy retouching can create.

There’s also a practical point: many fast or older lenses are softer at their widest apertures because designing a very sharp fast lens is difficult and expensive. So reviewers are often saying, “this weakness matters less for portraits,” not “softness is always desirable.”

In short: a soft lens isn’t automatically better for portraits, but mild optical softness can be flattering, natural-looking, and sometimes easier to work with than trying to fake the effect later.