Fitness walkers include many many former runners who have permanently injured their joints and simply can no longer run. If that's walking because they "aren't fit enough to run" then I guess its true!
As for myself- I sprained all my ankles and knees so many times as a child that as an adult running is out of the question for me. It produces immediately and significant joint pain. I do fitness walking alternating with my bike riding.

Personally, I think fitness walking can keep you just as "fit" (healthy?) as running. MORE fit, if you take into account the many stress injuries runners tend to get.
We already know that walking is great for heart and lungs.
I have also read that brisk walking is good for preventing bone loss (biking is not).
As to weight control- watching calorie and fat intake is just as important as the type of exercise you do. If you exercise regularly and are still overweight....well you need to eat better (and less) as well.