Its just all about what you want to do. Yes, you can take a 29er Hard Tail and ride gravel roads. What you deal with is a good bit more drag, The riding position is not the greatest for 30 mile rides. With a Gravel Bike, the 40mm tires work really well, very little drag compared to a 29x2.35 Maxis Ardent/Icon Combo. The Handlebars and geometry of the frame is more suited for longer faster riding. Just about having the right tool for the job. You want to jump jumps or go rock crawing
@Aquaholic , you go for a Full Suspension or Hardtail, If you want to do flips and spins you get an XFS, DVX, HE2, Eclipse, If you want to take long fast rides on dirt roads, some asphalt (Smooth or Rough) you get a Gravel Bike, If you want to take long fast cruises or run bouys, you get an SXR or SJ.......... You want to just cruise around the neighborhoods you get a cruiser bike, Just like if you want to take long slow cruises on the lake or bay, You get a 3D or a Couch......... But as long as your out riding some kind of bike, who gives a Schit. Go Ride