Is it bad form to call "ready to depart" when you know you won't be cleared yet? My local aerodrome is towered and has parallel hard and grass runways. Simultaneous parallel departures or landings are not normally allowed. On one occasion we had been instructed to "line up and wait" on the grass runway. While I was performing the pre-take-off checks, an arriving aircraft was cleared for touch-and-go on the main runway. I observed it on short final as I completed my checks, and called "ready to depart", even though I knew that we would not be granted take-off clearance until it was well into its climb-out.

My instructor later told me that asking for clearance when you know you can't have it yet is bad form, and that he would have told me not to, if he had seen the traffic (which had been in his blind spot). I can see how it could seem like impatience or poor situational awareness, but it seemed to me like it helps the controller by letting them know they can get me off the runway as soon as they're able. Additionally, I've been taught that in other situations it's good to get a call in as soon as I can and there is clear air (such as downwind and final calls).

I'd like to get some clarification on this. Is it bad form? Why: for one of the reasons I suggested, or because of some other reason? What other situations does this generalise to?
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.