As for your specific questions:
Q1: I see a lot of Taekwondo students using a low guard when sparring, is this advised and why?
There is a low guard and then there is no guard. Olympic style sparring uses no guard; two opponents just dance around and throw continuous kicks at each other. Any type of martial art that is involved in competition is heavily influenced by the rules used in the competition. The art adapts itself to the rules so that there is more of a chance for victory while competing under the stated rules. If you want to sin, you must use techniques that the judges will give you points for using, and avoid techniques that do not earn you points.
Traditional karate fighters use a low guard in competition because someone in the past determined that that was the way it should be done. Since both competitors use a low guard, any disadvantage to using the guard is equal for both competitors. Olympic Taekwondo rules discourage the use of hand attacks and kicks that do not hit vital areas with power do not score, so Olympic style sparring has evolved into foot fencing with the arms hanging at the sides, similar to Irish river dancing.