I've seen this debate multiple times on Runner's World's forum and decided I'd talk about my thoughts on it. As a consistent, 2 a day, scale watcher... I think that there is nothing wrong with it, provided you don't go overboard. I do think it could be unhealthy, depending on the frequency and the reasons.
But, I feel like the reason I do it is valid. In January of this year, I was 230 pounds. That's not ridiculously overweight, but it is enough overweight to be very unhealthy. Nate and I decided to start the C25K program and it became infectious. Fast forward to today... I have lost 30-35 pounds (depending on the day) and am very happy with how far I've come. That said, losing weight has never been my problem. I've done this multiple times. My problem is keeping the weight off.
So, now for the explanation of why (for me) I think scale watching is totally fine.
1. I do it for self maintenance. I never look at the scale and get upset to the point of unhappiness. It really is just a number and a gauge of how I'm doing that week/day.
2. I still have a goal of losing 10-15 more pounds. I need to keep my weight top of mind. It is the biggest reason I run... but not the only reason.
3. At some point I will be a weight that I will be compelled to stay at. And, I need a way of keeping there. I've been down this road too many times to do it all again.
Admitted reasons it could be unhealthy:
1. It can become an obsession and overwhelm you. I have totally been there. But, not right now. And, not since I've been running. Ever since I have been a "runner", I haven't worried so much about my weight. I knew that it would kind of take care of itself. When I lost weight before, I didn't feel that way.
2. The scale and weight that it displays is not a total judgment of fitness and overall health. But, it is one measure. Coupled with how your clothes are fitting, blood pressure, running times... it is a good addition to the mix.