For me it's, "I don't want to lift weights too often because I'll put on muscle which will slow me down" Unless you run the marathon or are already elite you could probably use any muscle mass you can maintain while running high mileage... Even in the marathon heavy squats are shown to help by improving economy.
I put on muscle mass very easily and I think it actually does hurt my running economy. I think I may be a natural rock climber/weightlifter as strength to weight ratio seems to my strongest attribute, and I can do 20 pullups untrained. I have a very lean build, but for example I can put on 5 pounds of muscle in a week just by doing some push ups and pull ups once a day. Should I really be lifting weights? Any advice would be much appreciated.