I got into mtb seriously a couple of years ago and decided from the off to ride flats. When buying shoes I wanted something that I could trail run in, walk the dog in, and ride in. I bought a pair of adidas terrex approach shoes as they are pretty good for all those things.
They have a solid base and good ankle support so they are brilliant for running in. They have some cushioning so they bear up okay for walking longer distances, and the sole has excellent grip levels (the toe on this particular sole also has a slick gripper apparently designed for bouldering) which is great for hiking on the rocks in the Lake District.
All these features to me seem great to cross over into mtb. The rigid sole performs just as an mtb specific shoe would for pedalling, the cushioning is great when you are in them for hours on end, and the grip levels help for security and confidence on the pedals, and also are great for hike-a-biking, especially the slick toe on the steeper climbs. Most of all, I even picked up a bargain, £45, where most mtb specific shoes will cost a load more than that (as an aside, I believe fiveten are now owned by adidas, so are likely sharing tech).