Not all OTTBs are crazy, but in my experience there are more "crazy" ones around than sane ones, although on a forum such as this you may not see it as much because many people around here are reasonably cautious about buying a horse.
Training has a fair bit to do with it, because I think that even if you find an OTTB with a good temperament, if he hasn't be retrained correctly (usually professionally) he can often default back to his old training, and breeding. A problem with the OTTB is that they are unpredictable. If you have a horse that misbehaves reliably you can learn to anticipate this behaviour and train it out, or at least try to, but with 90% of the OTTBs I've known and ridden, they are perfectly fine for a reasonable portion of the time, but then suddenly go nuts or be completely weird, then normal again, so you can't really do consistent training to improve them. This also means you go look at a OTTB, he goes great, a few weeks later a psycho etc.
You can get perfectly nice OTTBs, lazy ones, sweet ones which is all very well, but people talk about this as though these qualities make a horse good. I don't think that saying a horse is lazy or nice means that they are worth having. When people talk about QH or SH or Anglos or Warmbloods for most of these it is expected that the horse is normal. These are often purpose bred to have normal, stable and trainable temperaments where the OTTB is not.
Overall I don't think the OTTB makes a good riding horse. There have been many articles written discussing how their conformation can make them less than ideal for collection and general riding. Their typically high withers can make saddle fitting a problem, with most people just chucking a riser pad under the saddle to attempt a dodgy fix. The temperaments can be questionable, many develop soundness problems, have poor hooves, get in to trouble with fences and other horses (so do others, but I think OTTB are over-represented in this catagory). They tend to be harder to keep the weight on, harder to keep warm on have a low resale value. So basically the selling points for the OTTB are that most are athletic, attractive and cheap.
Really though, I don't think there is much that an OTTB does that an Anglo Arab can't do, or some of the varieties of warmblood, Irish Sport Horse, ASH and to be honest a QH would fulfill the needs of most OTTB owners. Or if you must, a purpose bred riding Thoroughbred. I just don't understand why people buy OTTB so often, they're not bred or trained to be riding horses, and there are hundreds of breeds out there that are. It just seems so illogical to me.
I don't see your logic here. Any OTTB that I have ever met that was as you described; erratic and crazy, had spells like that for a reason. Inconsistant ground training, pain, ulcers, or something similiar. Some are untrainable yes, but not because they're unpredictable. All horses are unpredictable. That's why we have to 'train' them and not just 'program' them like an automobile. They each have their quirks and their limits, and its our job to find them. Any horse, no matter how well trained it is, can blow up at any second for any reason and you might not expect it. If an OTTB is deemed untrainable, it is due to physical inability of the horse or the inability of a trainer to handle said horse, not craziness.
I can almost guarentee to you that the reason those horses that you rode would 'spazz out' without any apparent cause is because of incorrect training. As I pointed out previously, MANY OTTBs are not retrained correctly because we forget that they haven't been exposed to many things that your average colt would before training to ride. You have to start from the beginning to get a good mount.
I started Noah at 'weanling' stage. I pretended that he knew absolutely nothing and started with simple things like correct leading and tying, then I went from there. Even if it seemed like he understood something, I would teach it again and reinforce it anyways.
These are often purpose bred to have normal, stable and trainable temperaments where the OTTB is not.
I don't understand how an OTTB has a 'not trainable' temperment. I've never met an OTTB with a behavioral problem unique to only race horses. Pushiness, agression, etc, are all found in any breed at any time. These traits only show up in OTTBs more often because, once again, they have huge gaps in their training. Every OTTB that I have worked with was able to learn proper behavior within a short span of time given consistency.
Overall I don't think the OTTB makes a good riding horse.
While you did bring up some valid points such as saddle fit and soundness, an OTTB can
make a perfectly fine mount. Why else would they be the top choice for low-level eventers, dressage riders, and even rodeo people? Versality. Because they are not purpose bred for a specific job (such as a cutting or reining horse with purposely bred butt-high features or sickle-hocked legs) they can be used for many things and excell at it. Yes, many of them break down with artheritis early on and no, an OTTB is not very likely to make high level sporting again, but they can be nice
all-arounders. And not all of them have lameness issues. My guy Noah hasn't had a single lame day in all of his training, even though he raced for almost ten years and had nearly 60 starts.
Often an 'OTTB' is really just a 3 or 4 year old who raced once or twice, showed no 'wow factor' when it raced, and is for sale to be used for a different sport. These horses have not been ridden very hard and many of them are VERY sound. There are those who have had long racing careers, ofcourse, and those do tend do have problems, but an animal who has worked so hard for so long deserves a nice retirement as a trail mount or 'for fun' horse anyways, don't you think?
Here in Houston, over 400 Thoroughbreds leave the track every season to be sent either to slaughter or resold. Because of their abundance, they are cheap. Not because they are low quality. There are some very nice, healthy horses who are put down every day, and trainers try hard to find homes for as many as they can- but the reality is that some horses are going to have to be put down just because of the sheer numbers. OTTBs need second chances, and people like us come to give them those chances. Thats why we buy them. An animal shouldn't die just because it's not fast.
We bought Noah for $250. If he were to be resold, he would probably fetch us about $1,000 right now, maybe a little bit more. That's with one year of general training. But you know what? Even if he is shark-fin withered and is hard to keep weight on, he's the best school horse we have right now. I've NEVER once had him offer to buck, never had him jeprodize me, and never had him act out against the other horses or try anything stupid. On a ranch full of miniatures, ponies, donkeys, cattle, quarter horses, and saddlebreds, he's the most sane animal we have.