It's presumably in Pfizer's best interest to hold its cards close, eh?
I don't think it's that at all. I imagine it'd be in their best interest to let people if there's something that characterizes those who got covid despite the vaccine. If they showed certain behaviors, it would reassure people that the vaccine will protect them as long as they avoid these behaviors. If they had certain underlying conditions, it would reassure the rest of us that if we don't hae that condition, we're safe. Or if we do, we should protect even more. That is, I think eliminating the uncertainty caused by those few who got sick would reassure some of those who are hesitating to get the vaccine.
Unless, of course, there is nothing that characterizes those people. If it seems to be a totally random thing, then that might be less reassuring.