I like running on a TM, but I've just joined a running group and the coach says the impact of roads builds up bone density in the lower extremities. So I think the TM is good for building up the muscles in the beginning and on icy days but roads are good for building bone density. I started running just over two years ago and I notice the difference everywhere.
Bone density for MTBers is of huge importance. The difference between a bruise and a fracture on falls. Being banged up but on the bike the next day or in the hospital get metal put in. Probably every MTBer should be running and lifting heavy sh*t just as part of an injury avoidance program.
In a side note, there exist studies that suggest cycling lowers bone density, though I think they've only looked at it with roadies.