There are also down sides to leg protection, particularly on the trail. They do not allow the horses legs to cool naturally, possibly important over the hours of a trail ride, as opposed to an hour or less in arena work. They will also catch sand and grit and sticks and grass seeds that can make a horse raw.
I use them in arena work where there is crossover of legs, eg spins and two track work in reining. I use Davis boots because there is tough plastic on the outside that does not catch grass seeds and dirt, and neoprene on the inside. If the horse bangs one leg against the other you will protect the splint bone (thin vestigial bones down the cannon bone) from being broken.
Try riding through a grassy meadow in SMB boots - 10 minutes riding - 10 hours picking out grass seeds. Not practical at all. I have the same problem with hoof boots that are now almost unuseable from grass seeds in ALL the velcro as well as through the fabric. YUk.
So, for me, leg protection always in arena work, almost never on the trail (or in my case, not trail, because I ride around the farm with no trails).
The exception to this might be where a horse has some sort of deviation in its leg conformation that led to brushing or interference or treading on its own heels.