Personally I think mode changing on QoT is OK because she can move while changing. TB is not OK and he should also be able to move while changing. When I play QoT, I have no problems mixing Open/Closed skills. When I play TB, I go exclusively fire or ice, specially on small maps where you barely have time to change.
As Chaoticon said, playing a hybrid TB and switching forms without dying is not difficult. You just need to learn when it's appropriate to change and when it isn't.
The ability to move while changing forms would be completely overpowered. For example, switching from fire to ice would grant 50% bonus damage and activate a slowing aura on your opponent, all WHILE you were chasing them. This gives them exactly zero chance to get away from you.
Finally, on my computer at least, as soon as I hit change forms, my ability to move is completely locked. Issuing repeated move commands while changing just makes this dull error sound until the form change is complete. I have never canceled a form change - I can't.