There are certainly qualities and a level of experience that goes into being a good trainer. There are definitely situations that require those qualities and that level of experience to safely and efficiently handle. Even outside the "problem horse/colt starting" sphere, it takes experience, timing, and feel to teach more advanced concepts to broke horses. Most good, successful trainers spend time working and learning under a master, and this is where it shines.
To me, it is very difficult for anyone with a basic understanding of horsemanship and common sense to truly ruin a horse. Part of that common sense is knowing when you're in over your head and need to seek professional help or advice from another source.
Should someone who has only ever taken tourist trail rides buy a weanling to raise and train alone? No. In other threads, other members have posted about what they term "Disney syndrome", or something to that general effect -- city kid meets wild horse, they inexplicably bond, and they win some great event together, and everyone lives happily ever after, with no broken bones or bad habits. Some people incorrectly take this as a representation of real life, and then trouble starts.
I see a middle ground, perhaps someone who has owned well-broke to finished horses, taken extensive riding lessons for some time, and wants to move to a horse who has less in the way of formal training, or perhaps is a bit younger, and gain experience with a more difficult horse. With common sense, I can see that that working out well. Not a great deal different than moving to a lesson horse with more issues than the schoolmaster. Just matching horse to rider level.
A lot of extremely proficient riders, competitors, and professional trainers out there are self-taught in one respect or another. There's just a difference between a fairly seasoned rider taking on a measured and understood challenge and a complete newbie living out Black Stallion dreams. One has a good chance of ending well, one doesn't.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown