One important reason to be picky about conformation is that good conformation results in a horse who is just as usable at twenty years old as he or she was at five years old. Some horses are built to stay sound, and that is exactly what happens, whereas others are built to break down and...ditto. Check out the examples below.
Here we have two Thoroughbred mares listed for sale in the under $600 range. The one on the left is four years old. The one on the right is twenty four years old. Which would you rather have a baby out of?
Everything about the first mare looks uncomfortable. That shoulder angle certainly doesn't make for a long stride or a comfortable one. Look at how inflexible that neck looks. The back has a mild roach to it and at the young age of 4, this mare's front legs already have that "rode hard and put away wet" look to them.
In comparison, the second mare has a lovely balance to her. Her topline does show her age a bit, and she has a long head but I'm not very critical of that in a Thoroughbred. They aren't meant to have the same sort of head you'd want to see on a stock breed. Mare #2 has an overal look of quality that Mare #1 lacks. Her withers and croup seem to be exactly level, which is perfect. With her short back, lovely shoulder and overall balanced look, she would be suited for a number of athletic disciplines. She does have a goose rump and a high set tail, but again, I'm not as critical of that in a TB as I would be in a stock breed. Different disciplines, different priorities. Mare #1 looks very sweet, and I am sure she is, but her conformation faults doom her to being unable to stand up to hard work. I hope she will find a light riding home that will not overstress her. The ad says she is no longer breedable, which is probably a very good thing.
Creating horses who are not built to stay sound does a huge disservice to every horse breed, and it happens all the time because people look at their lame f'ed up mare and go "hey, let's breed her!" No. No. No. Unless she is lame truly through no fault of her own (i.e. an accident leading to an injury or somebody riding the crap out of her at age 2), don't breed her. One of the main purposes of controlled breeding is breeding OUT defects. If you can't wrap your mind around that idea, DON'T BREED.