Not every horse is a quiet, cruise-along driving horse. I mostly prefer to drive hot and/or green horses so yes occasionally I come across one that likes to have fits or buck! With time and patience most horses will "get over it." However, I like to have the safety of the check, which restricts some of a horse's silliness. It can be a good safety feature, particularly if you are driving a young hot draft horse that is 2,000 pounds! It's a lot of horse to just let have complete freedom in front of a cart.
A properly adjusted check, in my opinion, is one that is not so tight as it makes the horse fret (too tight and the horse will shake/toss it's head and possibly rear up in frustration) but tight enough that the check rein does not allow the corners of the horses' mouths to drop below the throat of the draft collar or the breastcollar. Most of the horses i have broke begin with no check at all while I ground drive, and then I begin to ground drive with the check rein on so they get used to it. As they get used to it, they lose a hole in the check until it is adjusted where it needs to be.
In a Park horse, the check eventually has to be fitted so the horse carries it's head at the highest possible level but still be able to flex at the poll and tuck its chin inwards. This takes a lot of time and care in training and help to build the horse's muscles to carry itself this way. Some trainers use a dumb jockey to further enhance the head carriage and hollow out the back.
The hollowing of the back creates a different movement than a dropped head and a rounded back. It is pure preference for the style of driving that the horse is performing in. Neither is "better" or "worse" than the other... it is a matter of preference.
Some horses, like my little stallion... are easy going hook-and-go sorts that could care less about the overcheck. I can check my stallion up in the show ring and he's fine... and he's safe and quiet on the trails that I can put it quite loose. My modern mare is much greener and hotter than he is, and has more energy than she knows what to do with and has "fits" because she can't gallop like she wants to in the cart. Once she settles down and we've driven out a few miles she is okay, but the overcheck helps ensure that both her and I are safe and she doesn't get too silly.
Driving is a discipline with lots of different tools and training techniques, and it's important to understand what they are so you can use or not use them properly.
In the CDEs you cannot have a check, and if the horses are properly trained to go without, it is a beautiful thing. In the Shetland Fine Harness the horses must be checked up ridiculously high to get the above-level action that they get, and when done properly is ALSO a beautiful thing.
I have great respect for both methods done properly.
Andrea