Isn't secrecy quite pointless for Bio-terrorist?
To answer the question in the title: No.  Without secrecy, it would be trivial for the BT to be caught, making them mostly ineffective.  Even a little bit of doubt as to where the BT is will often be enough to keep the players from attempting to capture them, since the moves would be wasted if they're wrong, and capturing often isn't strong enough to risk that.

In terms of why it isn't actually trivial to know where they are, you left out an action, which is probably the most common one.  The BT may drive between locations just like a player does.  This does not reveal where the BT is, and is your primary method as the BT of keeping the players from knowing where you are.  Also, when you discard a card, the players do not know if you are using it for a Direct Flight (going to the city) or a Charter Flight (going from the city).