I believe you should only be able to "vouch" if you put yourself at risk of getting scammed, and the other person didn't let you down. Because you're absolutely right, vouching for free stuff is pointless and meaningless.
But, you know what, we allow members to vouch for just about anything they want, so next time someone wants to trade with you and they have a hundred vouches from showing no trust, just turn them down and tell them their vouches are meaningless to you.
And if others want to not use their brain and get scammed by that same member, let them. You can't stop it from happening, sometimes people just need to learn for themselves the hard way or they'll never learn.
I made an announcement about vouching a while ago, I believe it talks about the same thing.