You'd be doing whatever you could to help your horse.
Its not only dangerous to force your horse to work through the pain, its also extremely damaging.
The latest research is shedding some interesting light on this often misunderstood topic.
Pain creates stress, and stress of any kind is responsible for 95% of all illness, sickness and disease.
What may begin as a simple twitch in the lower back, may in time become a hollowing, which eventually reaches a point when the horse almost collapses to the touch. All the while this increasing stress compromises the immune system, creates inflammation, places unnatural strain on joints, does psychological damage, and damages organs.
When your horse has an accident, or when the saddle hits a subluxated rib, the information about this injury is immediately transferred to the brain via the CNS. The brain is then responsible for deciding whether or not the nature of the injury is worth responding to. If its deemed a threat, the body will react in a way that will best protect the CNS to prevent or repair any damage. Your horse has no control over this impulse!
Pain does not necessarily mean tissue damage has occurred, but is instead the bodies response to whether action is required. For example a saddle hitting a subluxated rib - there may be no actual tissue damage that has happened, but in order to protect vital organs in that area the brain triggers a massive pain response, which comes in the form of bucking with a rider on its back.
So next time your horse plays up, check to see if pain is the cause.
The longer your horse feels pain, the easier it becomes to feel pain. This is because whenever the brain uses a specific neural pathway, it becomes easier (and easier) for information to be transferred along these lines. The more times your horse feels a certain type of pain, the less stimulation is needed to activate it.
You can see this in the case of horses who become increasingly aggressive when you groom them - as the neural pathways responsible for pain are activated on a regular basis, it becomes easier for them to experience pain itself.
Then pain association occurs...
When pain becomes associated with a specific activity (such as grooming, riding, jumping), simply seeing the activity can trigger an automatic pain response in your horse - you don't even have to apply the trigger! So when you have a badly fitted saddle that creates pain, over time simply seeing the saddle will trigger a pain reaction in your horse.
Which leads to sensitization...
When an area like the wither has become sensitized to pain, it will experience pain sooner, and with more intensity then before. Sensitization can occur anywhere on your horses body, but the wither, neck, pole, girth and hindquarters are perhaps the most common areas affected.
Leaving a horse in a pained state for a prolonged period of time will result in more pain, more sensitivity to pain, pain association, unnatural use of muscles and ligament structures, and many other challenges.
Riders who insist on using their horses before resolving pain are not only being unnecessarily cruel to their horse, they are also putting their own safety on the line.