What is the advantage of vouching - or can it only harm you?

How many people do you envision the average person vouching for? If it's less than 3, that's a problem.

The reason you vouch for someone is that you get the benefit of their contributions to the project and, assuming the person you are recommending is good, you get an improved reputation in the eyes of the person you vouchsafed them to.

Vouching for people is not a new concept, right? We do it all the time.

It usually ends with you getting banned or shadowbanned from vouching.