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Well yes, a bias frame is only necessary if you want to scale the dark frames.
Because a bias frame is fundamentally part of a dark frame, it becomes attractive to be able to subtract its influence prior to multiplying or dividing the time of a dark frame. In other words if I want to use a 5min dark frame on a 7min light frame, multiplying the dark by 1.4 will not be the same as taking a dark at 7mins because the bias value would also have been multiplied by 1.4. So scaling will sutract a bias frame from the dark, multiply/divide as required and add the bias value back to the resulting scaled dark value. So Bias frames are only required for scaling work.
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