This is true in many other situations as well, and indeed I personally have been in the same situation. Often the nuances of some business are best understood by those competing for that business. And let's be honest, for most of us the nuances of the VC world are poorly understood outside of actual VC companies. Sure, the basics are understood, but as with any business the specialists know more than the generalist.
So let's hypothesize that Marc understands this nuance, and wants to alert people to the issue. Clearly the warning benefits him at the same time as benefiting those being warned. Does he then speak out, or keep quiet?
To frame the question in a general sense, in situations where you have a credibility problem, do you go ahead anyway, or keep quiet and let others take the fall? What would you do? As far as i can tell it's a no-win situation.
I was in a similar position last year, and chose to write the warning anyway. I disclosed the conflict of interest - even though, like in this case the connection was obvious. To those being warned. Predictably I got a fair bit of negative response, but I like to think, if nothing else, it helped people at least take a bit more time to evaluate the situation.(I got some positive responses as well.)
So hats off to Marc. He'll get pilloried here, and elsewhere, but maybe one day if I go looking for VC funding, this is one more tidbit of knowledge that I'll have, which I probably wouldn't get anywhere else unless I learned it the hard way.
Full disclosure: I have no connection to Marc or any other VC. I don't know what his motive is, suffice to say that it could be either, and assuming the worst is not necessarily valid.