Rather than argue about whether being struck by a half-pound stick can cause pain, I'll say that it certainly creates the perception that a horse is being hurt. Perceptions matter.
As a culture, our sensibilities change over time. Things that were acceptable or taken for granted 50 or 100 years ago raise questions today. Dog fighting was a popular sport in the 1800's and had rules endorsed by the United Kennel Club. (see
http://en.wikipedia.org/wiki/Dog_fighting ) Today it's thought to be cruel.
Those close to horseracing see the use of whips as integral, harmless, and necessary to the sport. Much of the public doesn't see it that way.
I fail to see why whipping is necessary. If no rider had a whip, there could still be horseracing. In fact, there already is:
https://www.youtube.com/watch?v=vDagfA7qQ8s
https://www.youtube.com/watch?v=pzDM_0H_IEc
Okay, it's not exactly Saratoga, but it's horseracing. The first 3/4 of those races look a lot like a US race. The last 1/4 does, too, except for the lack of whips.
I don't know if whip use would have changed the outcome of those 2 races, and I don't care. The winners won fair and square under the conditions run.
There are a lot of reasons for racing's decline in popularity in the past 50 years. Energy spent defending the use of the whip could be better spent addressing some of those other issues. Defending whip use is ultimately going to be wasted energy. I'll be surprised if anything resembling hitting a horse with a half-pound stick will be allowed 20 years from now.
--Dunbar