Information gathering is crucial in every form of competition, from distance running and chess to business and martial art. Ignoring your opponent is in fact the height of arrogance and stupidity, and demonstrates a poor attitude towards your practice and sportsmanship in general.
Some argue that it's a shady practice, however those people are wrong. If simply knowing what an opponent can do gives you an unfair advantage, then they are relying on gimmick, and after a short period of time this will show. Studying their style however shows a respect for both their practice, and their ability leading to better competition on both sides. It is your ethical duty in a competitive setting to find the flaws in your opponents game and attack them until they are forced to improve. This demonstrates skill on your end, and forces them to elevate their own practice.
When two parties interact, they both have equal opportunity to learn and improve from their experience. It is your responsibility to adapt based on these exchanges and leads towards better art overall. If you share what you have gathered from these with your peers, then again, it only serves to elevate your art, and you can be certain that if your opponent is competent then they will certainly be sharing their experience with their group.
The other side of the argument is to say that you shouldn't adapt to your opponent, essentially learning nothing and changing nothing from any experience. This is ridiculous, especially because we as humans are incapable of doing this since it occurs on a subconscious level. It also follows that you must keep silent about anything that you have encountered, and allow others to continuously repeat the same set of mistakes without warning or preparation. If your are a coach or instructor this is doubly unethical, considering that this is your explicit duty.
We live in an age where the idea of 'secret tactics' are becoming laughably outdated, and relying purely on something novel can't cut it for more than a couple months. If the new tactic is sound, then it will continue working beyond that. Otherwise it will spread across the internet, and people will be able to watch videos or talk about it, and form counter measures to defeat it. This is the natural way that competition and conflict weed out ineffective methods. This is why every coach who is even half competent watches videos of their opponents before hand. It isn't underhanded, it is in fact the only sensible and respectable approach to competition, demonstrating dedication and respect for the spirit of your art and sportsmanship alike.