As long you apply a little discretion about it I don't see what the big deal is. EG: don't contact someone at the candidates current company unless that contact really IS you BFF and you know they can keep the conversation private.
Also, if you do get any odd reviews you'd probably also want to give the candidate room to explain. I wouldn't have a big issue with someone contacting random ex-coworkers of mine as references. There is one person in particular though that probably wouldn't have good things to say, but digging further on that guy you'd realize he's a complete nutjob. So, you'd just want to make sure you properly tempered all the feedback, but in that scenario you should see overall trends and be able to spot outliers.
I guess I'm also not highly opposed to this because I think most current interview tactics suck as screening methods, and everyone hand-picks their references anyway, so you kind of need to go out of band to get a really reliable reference.
I'll also add that I think it depends somewhat on the level of person you're hiring. I don't know if Halligan still personally interviews everyone coming into HubSpot. If he does, I'm pretty sure he is not the one checking references for a level-1 phone jockey. So, we're talking mostly about the higher-order candidates, key individual contributors and managers that he's doing this for. It just doesn't strike me as such a deplorable practice.
Furthermore, in my experience, references like to know in advance that they will be contacted: in my culture, any ethical reference is going to want to know who's asking, why, and whether they have permission to speak on the candidate's behalf.
Finally, there may be personal issues between the person you're contacting and the candidate that would make the reference biased.
Ultimately it seems pretty straightforward and professional to simply ask the candidate outright whether you can contact X, Y and Z as additional references (unless you happen to know these people well enough that discretion is relatively certain). At that point the candidate can say, "sure, but let me speak to them first to see if they're OK with that", or perhaps "No, Bob stole my wife from me and I hate that guy."
Why does this need to be blind at all? If there's someone that Brian would like to talk to, just ask the candidate, "I saw from LinkedIn you know Sheila. I'm good friends with Sheila. Would it be OK for me to contact her as a reference, or would that pose any problem for you?"
And I think it's also on Brian's shoulders to make clear why Sheila provides values. "Sheila is someone I've worked with in the past and value her opinion." Brian should also give you the option of giving her heads up, otherwise Sheila may view the cold-call from Brian as you dropping the ball.
All in all, just seems like a bad idea. And frankly, I'd really like to see the data that shows he's gotten better hires from this.