Yeah, I wouldn't call an agent out on the internet. Really just because it's not in me to do so. Rings of a rat, in my screwed up perception, and I just couldn't do it. I do realize this is not the optimal stance, because as you say, doing so could help other writers in the future. But it's just the way I was raised. Now, if someone did something horrible like stole my work or something I'd make an exception.
The writer in question must have been pretty good already to get to the point where such a reputable agent was seriously considering his work for representation. Tough break, and I won't say I haven't been tempted to jump the gun myself, but I'm respectful and try to look at others' pov. A saving grace maybe.
Just as writers, who may have a good story told well, get thrown into the slush pile where they may not belong, just due to the sheer number of subs and the conditioning of agents to almost expect a bad sub when they have to look through quite a haystack to find a needle, I think writers can be guilty of expecting every agent to be as dismissive as others they've subbed to--I mean, I know for myself, when i get a rejection that is personal, addresses me by name, and talks about the work with some depth, it feels like a win even tho it's a rejection. It's tough. Tough to always act appropriately, on both sides, when the odds are so long against either of us finding what we're looking for.