Um, darling Cricket, have you joined me at the mental ward?
I think the point of the pebbles is so that the plant's pot sits ON TOP of them, and therefore out of the water that is creating the humidifying effect. I can't see how having pebbles on top would help in this manner, yet it could be quite decorative. Let me draw you a picture as I am so fond of Paint at the moment.
P.S. Pressing F11 might help enlarge the spider mites.

Aw, Cricket, your rationale isn't faulty, and I'm glad that someone else in here is over-medicated too. Shoot, I'm no expert(see signature below), your theory could very well be quite plausible. We will have to let the Gardening Dooties guide us. LOL! I mean Dieties!!

As usual, I have to laugh my way through the smart-alec comments and references to post a professional answer. And now we have Amy's artwork, too. It's almost like you are saying, OK here comes Will with his boring answers, so lets lighten things up before he gets here and ruins it all.

OK , I'll stay in character and provide a sensible answer.

The purpose of the decorative stones is to help retain soil moisture, just like mulch does outside. The stones keep the water from evaporating into the air, so guess what? There is less air moisture and slightly reduced humidity. Now you could remove the stones, water more frequently, and keep the surface soil constantly moist, thereby increasing the humnidity marginally. Of course, the roots would probably rot from staying too moist, but at least the leaves woud die in a happy state of intoxication induced by the increased humidity.

Will The Bore

If any of this is unclear, look at Amy's sketch.