What if the VGA is not linear in dB?

I'm wondering..
What if the VGA is not linear in dB? How it affect the system performance?
Do we always need VGA linear in dB?

Linear in dB VGA makes easier to implement the AGC calibration.

Do you mean it's easier to analyze the AGC loop and its dynamics?

No. I mean the calibration of the AGC vs input/output power.