The short answer:
You should be doing chi sau, or fighting. No sparring.
The long answer:
I asked my WC sifu that same question a few years ago, & here's basically what he told me:
"Sparring" is what boxers do. We do WC. We're not boxers. We don't "spar", because that's what boxers do.
Every martial art has a freestyle exercise where both participants can practice all of their skills...Tai Chi has push-hands, Jiu Jitsu has rolling, Wing Chun has chi sau, and Boxers have sparring.
When Jiu Jitsu guys roll, they're practicing their strategy for a fight, but they're not training to fight the same way as a boxer. The moment Jiu Jitsu men stop rolling, stand up, & start throwing jabs, crosses, and hooks at each other they're no longer training Jiu Jitsu - at that point they're doing something entirely different - they're training boxing.
Conversely, the moment a boxer tackles his sparring partner & starts trying to get him to tap out, they're no longer sparring...they're practicing an entirely different fighting strategy.
The same is true for Wing Chun. The moment you stop doing Chi Sau, put on gloves, and start dancing around & trading attacks with your partner you're no longer practicing Wing Chun...you're doing something else...you're practicing boxing.
So Wing Chun doesn't spar, because that's what boxers do.
If you want to get good at Wing Chun, do chi sau. Do it at varying levels of intensity. If you & your partner can handle it, do chi sau at a very high, "I better get a mouthguard"-level of intensity. Do that often.
The other point I'll make is: either you're
fighting or you're not.
When boxers spar, they're not really fighting...it's a
mock-fight for practice. The same is true with Jiu Jitsu, Wing Chun, Tai Chi, or whatever.
Every art has a vehicle for freestyle practice of the art's fighting-strategy and its associated body of techniques. In Boxing, it's Sparring. In Wing Chun, it's Chi Sau.
Sparring has it's use as long as both parties know what each one is trying to achieve. You pad up and hit full force so that you can work on timing, quicker speed and positioning. However the thing to remember is to not have it end up being an all out brawl fest. That defeats the purpose of full contact sparring.
You bring up a good point, & I think it relates to my assertion of "either you're fighting or you're not".
Too often, people wind up doing something that's somewhere in between a real fight and controlled "sparring"....they're not
really going all-out 100% trying to knock each other out, but neither are they actually practicing Wing Chun (or whatever) strategy...they just wind up uncontrollably buffeting each other. Too often, when Wing Chun people try to "spar" it turns into this mess where they're not really learning about real fighting (because it's not a real fight), nor are they learning about WC (because they're not doing WC)...they're just wasting their time.
It winds up looking like really $h!11y boxing because they're trying to train like boxers without knowing anything about boxing while at the same time thinking they're doing Wing Chun.