I agree. You can't really teach a horse much in a race less than 6f. Its not the distance they are running, its the pace they are running at. At 6f or longer they don't all come out screaming and whipping and hustling. Pace is more realistic and more like what they will face the rest of their careers in races. Those short ones usually produce crazy paces and everyone has to scream and push hard just to keep up. And if you use one that short as a prep race, the problem is that they can be beaten 20 lengths and come out of the race with a negative view on racing, discouraged by the race.
A trainer i respect who has won hsi share of huge races told me that horses always remember the first race they ever run on the track more than any other. He said if they have a bad experience in their first race that they often never like racing or that it takes a long time to get them to like it.
Chasing or setting a wicked pace and getting beat a bunch of lengths probably isn't a whole lot of fun.
He also said if they learn early that they are supposed to gun themselves as soon as the gate opens that its very hard to correct that as well.