Mountain bike seat height is variable and depends on the riding you're doing, really. For fast, smooth trails, or a long climb, you'll want the seat at about the same height as a road bike for pedaling efficiency. But as the terrain becomes more technical, lowering the saddle helps give you more mobility in the cockpit. It gets it out of your way so you can move around easier. For big descents, dropping the saddle all the way down helps you get behind it.

In fact, you may even adjust it during the ride depending on the conditions. That's what the QR saddle clamps are for, and they make dropper seatposts so you can adjust seat height on the fly.