I've got a handful of spinnerbaits, some with ball bearing swivels, and some with cheap swivels. Other than not spinning as nicely when spun out of water, is there any difference in the water? I can't tell a difference just by looking when testing them close to shore, but perhaps I don't know what to look for.

With ball bearing swivels the blades will spin when dropping through the water. I think thats important. 

Just gonna tag our resident spinnerbait nerd @LrgmouthShad

One of the biggest differences is ball bearing swivels will get those blades to start turning as soon as the bait hits the water.  In addition, you can retrieve the spinnerbait faster without it rolling over on one side.

@Glenn covered it. 
 

I use ball bearing swivels on all my of my spinnerbaits for the same reasons mentioned here.  I would never throw a spinnerbait without a good ball bearing swivel.  That said, I sure wonder why the bass will hit my friends cheap spinnerbaits with rusty old barrel swivels.  Don't the bass know they should only hit the expensive ones?   Like many things in fishing, I may care more about the details than the bass do.