--Kim!! VERY well stated, and very much on target! (I would, however, take a different view of a running martingale(see below).
I believe pretty much everyone who shows A/R knows that the breed registries require checks--so yes, if showing there, one has to use them...
And yes, if driving on a road, there is unlikely to be anything growing underfoot that is tempting to eat. However, it should be remembered that some of us deal with different driving conditions than others....here, the "country" roads have too many rocks(just the right size to cause stone bruising). Most of my "outside" driving is done along the highway right-of-way, and although the highway dep't. does mow occasionally(VERY occasionally!)-plants grow (gasp!), some of which are edible. Ergo, a comfortably adjusted side check IS a help, in this situation, to help prevent attempts to 'graze'. As for martingales-I ordered my first Lutke harness with one, as "everyone" seemed to use one. I found that they were of no benefit, and have not used one for years now. Recently, a friend, who owns a gelding SON of Rhotens Little Dandy, a horse that is a lovely, ground-covering mover, said she thought she'd get one, because she sees "everyone" using one...I just told her, "not everyone"-but, she will probably buy another harness, and get a running martingale, because she does not have enough confidence in herself or in this good horse(though he had NO non-arena experience, and has been a challenge to her in many ways, because he was one of those whose early "training" was to get him showing "yesterday" , as it is with so many...), that it isn't that gear that does the job, it is the genuine knowledge, understanding, and patience of the trainer.
And-although I understand the "accepted practice" explanation, I would respectfully suggest that just because a practice is "accepted" here or there, does NOT make it UNIVERSALLY accepted--nor necessarily a good practice. Breed shows, even if similar ones are taken all together, likely make up a minority portion of the much wider world of driving-i.e., what is acceptable in the UK(where there seems to be a LOT of interest in the driving of horses)-and even what is "accepted practice" within the American Driving Society. Breed shows,at least in the US, are pretty much about "roundy round" the ring( and that is what I did, for 20 years, mostly in the AMHA ring); thankfully, I have learned that there is MUCH more to driving than that!
I am in agreement with what MiniHGal said regarding having a horse driving in a couple of weeks. Possible? Yes, but with a number of caveats--and the knowledge and experience of the person doing the training have SO much to do with it-that's all I will say on the subject.
Are there breeds where gingering is actually allowed? If that is so, I find it....surprising.