There is a lot more than gut judgement involved in deciding to give somebody I just recently met a large sum of my money. I don't let other VCs dilute my judgement but I certainly listen to them when they have more knowledge of the relevant facts about a company, its people, its product, or its market.
Note that I'm an angel investor, so I try to make sure the cost of due diligence is proportional to the amount I'm investing. If I were a fund and writing million dollar checks, I would spend the time to do all the due diligence myself.
There are a bunch of investors out there that I know, like, and trust.
Also slightly less importantly, there are investors I don't necessarily trust.
The composition of the syndicate is an important signal.
But I think the salient point is whether the investor is asking who the other investors are so they can cooperate with them, compete with them, or collude with them. And that depends on the investor, not whether they are an angel or a VC.
One thought that comes to mind is if you mention that you are also talking to "Sam " then "Bob" (who you are having a conversation with) can start to game Sam, or Sam can game Bob (when Bob mentions it to Sam later if they know each other).
So in terms of general business "loose lips sink ships" this is something to avoid.
So either party can play mind games with the other. And depending on the relationship of the two this could create a problem.
Other thing is that this gives the VC you are pitching a chance to drop things in your mind about the other VC that might affect your judgement. "Oh well he's a great guy but the one thing you need to consider is..."
My guess is PG might feel that the type of people in YC don't have enough experience to manage this situation properly so it's really similar to an attorney saying "don't say anything let me do the talking".
But if one VC knows who the other VC is, then collusion is simple and clearly in the VC's best interest.
The compromise position here is being honest but guarded - ie, especially if you are talking to one of those above - As a founder / CEO's, come back to the investor here and say we're talking to a couple of top-tier firms, which gives the signal effect to the investor that they need to sell / convince quickly while maintaining your leverage as an entrepreneur.
Why not? (Unless I missed it, the article doesn't explicitly say.)