Why don’t you out the sites you advised to purchase links? I mean, you clearly stated:
I’ve encouraged some of our clients to purchase links in the past and I suspect I’ll do so in the future (though we generally try to be extremely careful about it).
So, obviously you know of some sites that must both rank well and use the tactic of buying links, and know them well enough to illustrate your point much more clearly than with some random sites that are merely showing “a lot of questionable links”, don’t you?. Plus, since it is obviously your honest opinion that it doesn’t hurt anything to do so:
Clearly they had already seen the Forbes issue and their content still ranks great – Forbes has a massive amount of search traffic and rankings from Google.
Why not just use your clients sites in the first place? Doesn’t that make more sense than using some random sites that you really aren’t all that familiar with? Since it’s ok to do it anyways, I mean.