On a mechanical device, the motion is likely to be nearly identical from step to step (or cycle to cycle), leading to repetitive stress on specific muscles.
Outdoors, things are far less uniform, spreading the wear and tear around some more, as the motion varies from step to step.
That's my perception of it.
I think Mark's point is valid. I think it's also why doing things outdoors is often better exercise than doing things indoors. More muscles get stresses in different ways leading to overall better conditioning. At least that's my perception. I have seen quite a few people who are regulars in the gym not fare all that well on the trail.
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