It is automatically diminished. By you being able to burst anyone else, including the original caster.
You ever played the game Mafia? The Doctor/Angel role is able to choose one person to save each night. If they guess wrong, they did nothing, and the person they didn't choose dies. They're also vulnerable to being killed, unless they choose themselves (and even then, they can still die to vote elimination).
It's the exact same thing with shielders. They can only really save one person. If Janna shields her ADC, burst her, her mage, her assassin, or any other squishy on her team. One person is useless unto themselves, with the rare situations of a champion that's gotten too strong for anyone to handle.