I strongly, strongly believe that once you are capable enough, you need to sit your butt on as many horses as you can.
I was a member of the state MG squad. In MG competitions, there is such a large amount of ponies needed, that teams don't take their own horses. The cities/areas that are hosting the event organise a 'pool' of ponies. The ponies are split into groups of 5 - Four used in each race with one reserve. These groups are called 'brackets'. A bracket swap is done every three races -Every team has a new lot of 5 horses every three races. They have roughly 2 minutes after the bracket swap to test the ponies and decide who is best on what, then they go out and race.
In preparation for this, our squad was made up of 15 riders. We did the first race on our own horse, and then every race, we swapped a horse to the right, so by the end of the day we had ridden 15 different horses, often more than once. We had 30 seconds out back to test them out before going into the race.
There is NO better way to imrpove your riding and improve your ability to assess a horse in a small amount of time. I wouldn't be half the rider I am now without those years in squad riding ponies from well broke, experienced games ponies, to never seen the equipment, nervous and rearing ponies. Ponies who dropped shoulders into poles and barrels. Ponies who took off when you tried to vault. Ponies who shied away from changeovers and ponies who ran into them. Ponies who wouldn't stop and ponies who wouldn't go.