I have used a treadmill at various times in the past for some of the same reasons that people have already listed: I could get a run in when my kids were younger and were in bed (since I couldn't leave the house) and also when the weather was poor and I wanted to do more than plod through the snow (although that can be fun too).
I used the treadmill extensively a number of years ago when I was getting ready to run Big Sur which takes place relatively early in the year for an Edmonton runner. I did a lot of my speed workout and hill workouts on the treadmill throughout the winter however, one thing I hadnt considered is that for hills on the treadmill there is no downhill portion of the run. When I got to Big Sur I did great on the uphills but when the next long downhill came everyone that I had passed on the way up blew by me on the way down. One friend even went so far as to compare our mile-splits to the course profile just so he could laugh at my total pathetic downhill skills. Specificity of training at its finest.
I also used it a bit for heat training for WS a few years ago when I would do the "overdress, crank the heat, run indoors and sweat" form of heat training. Again, better than nothing when its only single digits outside and you want to get ready for a hot race.
Its a tool like lots of other things; use it right and its great.