Dave had a bad workout this last weekend - his first since starting my training program. He'd been crunched for time and was trying to squeeze the 80 minute run in late at night and so ended up at the health club on the treadmill. Running this long on a treadmill sounds like torture to me and I can't imagine choosing to do it. In fairness though, I spent last saturday on a stationary bike for an hour and forty five minutes (although I certainly didn't like it). I routinely choose to do my shorter workouts, however, on the trainer or the treadmill and view these options as extremely valuable tools for a program such as mine.
Why? Because they create and allow for competition and highly controlled efforts with constant feedback. I can choose the pace for my intervals ahead of time and have a target to shoot for. I don't have to factor in wind, road conditions, detours, etc. The number of variables at play during a given workout are reduced down to possible variances between machines (probably slight or even negligible if you get the same machine) and factors that are stemming directly from the person doing the training. It is these latter factors that are most important and which are able to be isolated by consistently reproducible training. Improvement can be tracked very closely and immediate modifications can be made to a training program based on each individual workout. If you're time-limited enough to be hell bent on maximizing fitness in three hours a week, these things begin to matter.
Another benefit of machines is that they can effectively be used to overcome low motivation or to have a good workout despite it. A machine, especially when used for interval work, more easily creates a 'challenge to be met'. Once you set-up that first interval, all you have to do is hang on for the ride. The belt becomes an adversary - and if grabbing the bar is not an option (or is only an option if it's immediately followed by the expulsion of some type of bodily fluid) then it'll be hard not to walk away from your treadmill glad that it was such a worthy one. And I'd bet your low motivation is long gone to boot.
Of course most people groan at the thought of treadmill/stationary bike work and I don't blame them. I vastly prefer running on the 8 lap track at my health club to a treadmill and it offers many of the same benefits - a controlled environment and nearly continuous feedback - but I get to actually move through space. This brings me to the idea that although, as I just mentioned, cardio machines can be super useful - a well balanced workout plan will not rely on them entirely.
Running outside (or even on a track) is not like running on the treadmill, and unless you plan on racing on one, you'll need to make sure all your indoor effort translates to outdoor results. It's not only physically harder dealing with the elements, but there are also important mental components that won't directly transfer beyond the walls of the gym. For example, pacing needs to be worked out. Perceived exertion and HR levels may differ significantly between indoors and outdoors, usually in a way that favors the one done the most (i.e. if you run inside a lot, it will end up feeling easier than running outside at the same pace). Ideally you'd develop a balance so that this difference is minimized and workouts can be transported inside or outside with similar results. In addition, the types and forces of will required to maintain a pace on the treadmill versus out on the road are two entirely different animals. It's analogous to what's happening with the guy who's barely hanging on at the back of the lead pack rather than out front, driving it. Believe me, they're both hurting (probably badly), but what's going on inside their heads is quite different. When you're on the treadmill, you're the guy in the back. I don't always want to be in back, do you?