All my bikes had a flat seat

[Gene]

but now it seems most of the bikes I see have a "hump" starting about halfway down. This has puzzled me, and to be truthful, I don't care for it. I don't think a passenger needs to sit higher, but I had a thought. When I had my Suzuki 500 Titan, flat seat and all, I was detailing it, and decided to Armor All the seat. huge mistake! After that, I decided to go for a ride, and the bike took off without me, it was too slippery. Perhaps if my Suzuki had had the "hump" like my Honda, that wouldn't have happened.

[mollusc]

My NC has a hump because the rear half lifts up to get to the fuel tank.  But I think the hump is mostly there to give you something of a pocket to sit in.  As you say, sliding around isn't great, and would certainly affect your ability to control the bike if it happened at the wrong time.