Also, even if Cowl is Simon, and everyone would no longer give him help, knowing that Cowl is Simon would still enable them to better defend against him. People know him, people know what he can do, to some extent. As Cowl, he's a cipher, and harder to predict and defend against. Also, if he was planting metaphorical landmines in the Council, outting himself as Simon would make the Council more wary of things he did.
I personally do not think Cowl is Simon, but if he were, there's still PLENTY of reasons to hide that fact.