I could be perverse or naive, but as a long-term holder, I view the lack of disclosure as working in my favour.
It's a 7.4bn dollar fund, with typically 8 to 12 holdings, so could easily be a billion dollars a go for the biggest holdings. So if the market had too close a handle on what PSH is holding it could anticipate Ackman's moves, so the fund trades stocks at a worse price, impacting us as investors. It's exacerbated as well by Ackman's reputation, and the fact he probably won't be trading diddly amounts.
If anything major does happen we soon find out anyway, like the Netflix purchase. And when the market did find out about this, the Netflix price went up. And it goes up exactly because of the announcement that Ackman has bought in.
So Ackman is I think playing the disclosure game to my advantage, twice. Not only is he being cagey about what he's holding, he also controls the information flow on his new deals. So he gets a cheaper entry price on Netflix, plus a pop when he does tell everyone about the new investment.
Finally, I'm in the fund chiefly because I want Ackman to pick the holdings, rather than me forming my own view on them. And the other big point is that the insiders (including Bill) all have big skin in the game, so their interests are well aligned with investors. (Other than creaming off big performance fees, but that is another debate....).