To deny a clearance because it increases workload due to no plan is pretty poor.
It would be a matter of priorities for the ATC, as it should be.
You're not to know what is happening in the ATC's environment and going on behind the scenes. To take details over the air and enter into Eurocat is a distraction from everything else that is happening. Usually that's manageable, sometimes not. A FPL in the system is the smart way of working in the system.