There are two types of mouth rinses as classified by the Food and Drug— therapeutic and cosmetic. Therapeutic mouth rinses typically contain fluoride and have been proven to be effective in helping to prevent cavities, plaque and gingivitis.
Cosmetic rinses are utilized mainly as a treatment for bad breath, and to help reduce the amount of bacteria and/or food particles left in the mouth after eating. These types of mouth rinses do not effectively treat gingivitis.
A therapeutic mouth rinse is a great addition to your oral hygiene regimen, and can definitely benefit an individual who my have trouble brushing their teeth due to a physical disability or condition, such as arthritis.
However, it should be noted that mouth rinses alone are not enough to combat plaque. Regular rinsing with water and using a fluoride toothpaste have been found to be just as effective, and should not be dismissed in favor of mouth rinses.