The work I have done has always required more than just gaining access to the network. The attitude I have encountered was that they didn’t like it if you could break into their wired network, but they also didn’t see it as the end of the world. They were worried more about what you could do after that, as far as stealing sensitive data. If you could access their network, but from there you were “stuck” because they had every box locked down and had a very attentive security in place, what could you do? Surf the internet? Start spamming? Yes that and some other things, but usually not for long. The fear was more about “hit and run” attacks.
I have actually, although rarely, encountered networks that you could breach, but from there you couldn’t do much as far as stealing data in a stealth mode. Sure you could do some malicious damage and that’s why I have always warned not to take even the slightest breach for granted. But taking down a network to prove that point is not part of a responsible pentest.
In my experience it’s very important to try and “own” the network after you have slipped through their wi-fi. Plant your flag or retrieve the file they purposely placed for the test and there will be no doubt that you were there. That’s important, because some admin will try and “play down” what you did so that they don’t look bad.