I don't really agree. I won't vouch for an asshole, but I'd much rather vouch for someone who is reserved and hard-working than overtly friendly but lazy or incompetent. I don't know of anyone I trust that feels different on that (the topic has come up many times).

I think a lot of the debate here lies in the gap between "vouch for" and "recommend".

It's not just whether people would choose friendly incompetence over reserved hard work, but whether the reserved hard worker is going to be a name anyone mentions when it's time to find a candidate.