Have just got home after 14 days on a cruiseship - and have come away with an appreciation of the use of a treadmill as a complimentary tool to use for running training. I did also try out one (once) on a previous cruise in August and came away thinking "not for me" . But this time I persevered and did about 6 sessions - which I enjoyed and feel very good about.
The biggest benefit for me was the ability to set out to do a certain task and to complete it to the letter of what was required - especially as a HRM user. One big problem for me is pacing myself to achieve a certain training requirement - eg that could be to run for a set period of time at a certain heart rate. Out on the road, I find that the hills and my own inability to pace myself causes my HR to go all over the place. But on the treadmill, I was able to to do short and long "threshold runs" at a specific required heart rate. I used it to simulate "hilly" threshold runs, and high HR intervals for set periods of time. The machine was set for a maximum of 20 minutes use at one time - so this meant that I was able to undertake strenuous tasks for a 20 minute period - a time which is really quite short to what I would normally do on a road run. I am thinking that from a cardio point of view, because I don't have to worry about the leg muscles side of things while using the treadmill, I can improve my breathing and increase my endurance at threshold/tempo speeds. I am thinking that long slow runs on the road - and faster ,shorter runs on the treadmill would be of great benefit to me.