Given that, would it (gliding flight) confer advantage to a dragon, fighting humans, given they are both trying to kill each other and the human band (at around 5 in number) has crossbows, which can wound and potentially kill dragons? The outlined situation is just a point of reference.
An advantage as compared to what? Walking?
The answer is: yes, flying is much better than walking!
You say that your dragons are "roughly at around the size of a large horse" and have gliding abilities similar to Quetzalcoatlus northropi. So let's compare the numbers on a horse, a Q. northropi, and a Grob G103 glider.
- Weight: Horse: 800 kilograms. Q. northropi: Unknown. Glider: 500 kilograms.
- Length: Horse: 2.5 meters. Q. northropi: 8 meters. Glider: 8.1 meters.
- Wingspan: Horse: 0 meters. Q. northropi: 10 meters. Glider: 17.5 meters.
Let's start by assuming that your dragon's weight, size, and general aerodynamic characteristics are identical to those of the Grob G103. What can your dragon do?
Well, to start with, she can fly around at 100 km/h, while only losing 75 centimeters of height per second. This means that if she starts at a height of 2,000 meters, she can fly at this speed for about 45 minutes without expending a significant amount of muscular effort. That's great for flying over humans without getting hit: if I have to go near humans armed with crossbows, I'd much rather be in the air, flying at 100 km/h, and turning randomly, than on the ground running.
And that's ignoring the fact that on a sunny day, your dragon can find columns of rising air (produced by warm spots on the ground) and use them to stay up for hours.
But can a dragon use gliding flight to kill a human? Absolutely.
The maximum permissible speed of the Grob glider is 250 km/h. (Any faster, and it's at risk of shaking itself apart.) It's easy for your dragon to attain this speed: she just has to dive, starting from at least about 500 meters up. Having attained that speed, a nice and easy way to kill the humans would be just dropping some rocks on them. Humans don't stand up very well against 250 km/h rocks.
After having dropped rocks on the humans, your dragon can climb back up and do it again. And that's without using any additional energy. But how many times, exactly?
If there were no drag on the dragon, decelerating from 250 km/h to 100 km/h would allow her to climb 400 meters. I don't quite remember enough from my calculus classes to figure out the exact amount of drag, but I did a couple of back-of-the-envelope calculations, and it's reasonable to guess that after climbing and doing a second bomb run, she'll have about 280 meters' worth of kinetic energy remaining. Subsequent attacks will use less energy, so she may have 180 meters' worth of energy after the third, 100 after the fourth, and 40 after the fifth, possibly leaving her with enough energy to make a sixth dive-bombing attack.
Let me repeat that for emphasis. Gliding flight will allow your dragon to "dive-bomb" the humans about five or six times before having to land.
Now, a wingspan of 17.5 meters gives you an awfully big dragon, and I don't know if you could get away with only 500 kilograms for an animal that big. But let's cut both of those numbers in half, giving you a wingspan of 9 meters and a weight of 250 kilograms. That's very similar to both the wingspan and estimated weight of Q. northropi, so you're in business. What effect will this have on the dragon's aerodynamic characteristics? Slim to none! Reducing both the weight and the wingspan by proportional amounts doesn't have much effect on aerodynamics, so all of the above numbers are still valid, with no changes.
My advice to the humans, by the way, is to stand near some trees.