Is it bad form to call "ready to depart" when you know you won't be cleared yet?
Informing the controller that you are ready to depart and requesting a takeoff clearance aren't the same thing.

It isn't bad form to inform the controller when you're ready for the next phase; if the controller isn't ready yet, you won't receive a takeoff clearance, but the good news is that now the controller is ready to get you going as soon as possible since your intentions have been declared, and she also has some time to think about what will happen next.

It opens up time for the controller to plan and coordinate.

Situation A:  You wait until the landing traffic is clear of the runway before you state that you're ready for departure, and then the tower clears you; you takeoff.

In this situation, the tower had a short time to react to your request and possibly plan other traffic around you.

Situation B:  You state that you're ready for departure, the tower acknowledges and requests for you to hold short for landing traffic. The traffic lands, and then, the tower clears you; you takeoff.

In this situation, the tower could have altered other inbound traffic to provider a bigger safety buffer for you due to the increased time given to asses the current traffic conditions at the airport.