I think if the fuse blows, the bias tap becomes the new center tap, and the bias voltage could theoretically go as far negative as the HT voltage was positive, destroying the bias circuit. You can argue that the HT winding might be floating, but I argue that if the fuse has just blown, it's probably because the HT is shorted to ground.
It always struck me as a really stupid place to put the fuse, and I have never put it there in any of my own circuits. It doesn't provide any protection against a shorted diode, any more than putting it in the positive rail after the diodes would.
I don't know why the fuses are different ratings, but if the designer was unfortunate enough to put it in the center tap without considering the bias circuit, I guess he is capable of choosing the wrong rating too. It's usually 500mA for a 50 watt amp and 1A for a 100 watt one, as they draw around 300 and 600mA flat out respectively.