Arthritic pain leads to decreased activity, decreased range of motion, and accelerated loss of muscle (rheumatoid cachexia). This leads to lower energy levels and an increased likelihood of injury. Serious injuries from falling will lead to a life cut short with a greatly diminished quality of life. This decrease in activity also leads to an increased incidence of cardiovascular disease.
It doesn’t have to be that way. According to this Arthritis Foundation article, Strength Training is Essential for Arthritis, strength training will: reduce pain, increase range of motion, burn calories, and boost bone density. The catch is finding the type exercise that will strengthen muscles and not exacerbate pain.