You may as well ask why bother having Knowledge skills at all, if it's incumbent on the players to dope things out through trial and error. Players put points into those skills to represent their characters' expertise, and they should get something worthwhile for the investment—especially if they use the information they get to drive how they roleplay the situation, rather than use it to make the best metagame decision.

It's up to the GM to moderate what information the players receive. The fault in your first example does not lie with the player who attempted to use a skill to get information relevant to the adventure; it's the fault of the GM who let that one roll give so much info that there was no tension or mystery.