So, we can all be in agreement here. Chi Sao is not sparring or fighting and I have not claimed it to be either! When I started, training would be half an hour physical training/warming up, one hour of forms and drills training and tha last half hour semi contact sparring with mma type gloves. For everyone, including beginners. Anyone who visited our school was invited to spar with us. One time only, did one of us get suckerpunched by a guy who could not deal with the amount of strikes and blinly struck my friend with a hook. Mind you, my friend was friendly sparring and did not cosider this to be a real fight. The outcome would have been much different!
As for the boxers and kickboxers, yeah we sparred with them also. We were invited to the MP training school to spar with their instructors. Chased them all around. Were considered to be too agressive for them to put it in their training methods. PB and WSL managed to show us their superior skills though.
Nowadays I study PB Lineage with less emphasis on sparring. In a way, I miss the old days, the sparring I enjoyed a lot! Wouldn't advise it though for at least after several years of VT Training. As pointed out, there is a danger of chasing hands and forgetting about body structure entirely.
My final point about the thumbs or throatstriking is this. My height is below average here. In a real fight, chances are my opponent will be larger and possibly stronger and also, maybe even a skilled fighter. It takes a lot to get me to fight, but when it happens I fight to survive and will use all my skills to end it as quickly as I can. This includes eye gouges, knee stomping and if taken down biting a pice out the opponent.