But going light and friendly just doesn't prepare you for the agressive reality of punches and kicks intended to take your head off. It's a whole different thing.
For me there's no difference between light sparring and hard fighting, other than the speed and force of the punches and I'm saying this from the perspective of the street fights I've been in.
Punches, kicks, take downs and defenses, still flow in the same direction in the same pattern. The flaws that you have here will be the same flaws that you have in full on fight.
If you have a good coach and training partner then those flaws can be addressed and corrected without having to be at an intensity where you are trying to knock someone out. If someone can hit you in light sparring they can hit you in hard fights. The only factors that will change that is speed and power.
The only thing that people need to really pay attention to is the context of sparring. For example, I shouldn't expect my push hands sparring to be enough to prepare me for a match where there is punching, kicking and grappling going on. I have to make sure that my sparring is in the same context that my fighting will be in. Don't do point sparring tactics if the fight is grappling because they are two different things.
If all you do is light sparring Wing Chun against Wing Chun then you will fail when it Wing Chun vs Boxing. If you do hard sparring Wing Chun against Wing Chun then you will fail when doing Wing Chun vs Boxing in a full on fight. The outcome is the same and the intensity of the sparring doesn't matter.
If you aren't training techniques that you can use to cause damage to your opponent then hard sparring won't make any difference.
I have never done hard sparring before in my life. Everything has been light. You may look at some of my videos and say that some are hard sparring, but in reality and for me, it's light sparring because of the conditioning that my body has. It's not light sparring for a beginner. The punches and kicks are harder but they are never thrown with the goal of trying to knockout my opponent.
Now if you are sparring so light where you don't care about being hit, then it's that stuff that doesn't help. Stuff like that will degrade your ability