A return loss of 10 dB is considered acceptable for many applications. Specifying bandwidth where the return loss was 3 dB would generally not be a good idea, as such a return loss is unacceptable for most applications. Conversely, specifying bandwidth as the 30 dB return loss value would generally be a bad idea, as 30 dB is unnecarrily good for most applications.
What is an acceptable return loss depends on the application, but 10 dB is not an unreasonable number. Hence the reason its often specified.
Obviously for most resonate structures (e.g. antenna), the resonate frequency is important. One would be less interested in the resonate frequency of an attenuator, since that will be well outside the operating range of the device.