Metal mounts are generally able to withstand wear and tear better than their plastic counterpart could. This is especially so for higher end lens that are heavier because of increased amount of glass elements and/or heavier, sturdier components that are used in the construction of the lens.

For example, a Nikkor 70-200mm f/2.8 weighs around 1.4kg. If someone were to hold the camera by the body alone without any support on the lens, this could lead to potential damage in the mount area. Both the DSLR body and its lens could reach up to a substantial weight and the only part that is holding it together is the lens and camera mount. The camera mount is usually made of metal, while lens mounts are either made of metal or plastic. As the proverb goes, a chain is only as strong as its weakest link and the link in DSLR is the mount.

This is just an extreme case example. In most cases I believe (I have not yet stress tested any metal or plastic mounts), the camera maker/lens maker would have made the lens mount with sufficient strength for daily use. Reviewers mention the metal mount so that users can have peace of mind when:

1. Heavy lenses are coupled with a metal mount instead of a plastic mount (for durability purpose).
2. Lens that comes with metal mount (gets your money's worth).

That said, it doesn't mean plastic mounts aren't worth anything. It helps to reduce the weight of the lens to reduce the stress on the user during extended use (covering an event or during travel). It could also possibly mean reducing the cost so that prosumers can get their hand on decent lens without selling off an arm or leg without compromising too much on the quality of the glass.

Alas, this is the end of my opinion. Do correct me if I had said anything wrong. Just a hobbyist's opinion on this matter.