First of all I suggest people to wear a mouthguard in EVERY training situation with partner contact! Even if it's only a warm up game. You could always accidentally run into each other. I wouldn't let people spar at all without a mouthguard. The most important aspect is of course to protect your teeth (which don't heal like a broken bone and are really expensive to get fixed if broken) and reduce the possibility of concussions, but also to protect you from those little, non serious but pretty annoying, injuries with inflammation on your mucosa when pressed against the teeth due to an impact. Another factor is of wearing the mouthguard in training as much as possible is to getting used to it so it doesn't disturb you while fighting.
So, about the Shock Doctor mouth guards. They are considered amongst the best in the "boil and bite" category. A custom made mouthguard from the dentist is usually still better, as the dentist can make it fit more exactly to your teeth, but it's also a lot more expensive. The best kinds of mouthguards are probably those like in this clip:
The brand named in the clip is definitely not the only one to make such a type of mouthguards. I've seen other brands around who do a similar multiple layer type of mouthguards. I recommend this type of custom dentist made mouthguard if you're aiming seriously to compete in the sport.
For regular training the Shock Doctor mouthguards are fine. I've had good experience with their Power Gel Ultra
. It's tight fitting (when boiled and bitten correctly) and very low profile so there is no problem with breathing and you're still able to talk reasonably well. But beware, they've somehow changed the fitting gel. I wanted to buy me a new one and the fitting gel was very soft, so the mouthguard easily falls out. So if you want one of those, look out for the old green colored ones and avoid the new blue colored. The same problem exists with their Ultra STC model.
If you buy a boil and bite mouthguard like the ones from Shock Doctor, follow the instructions for fitting carefully. I've seen lots of people who didn't so that their mouthguard didn't really fit and easily fell out.