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I strongly disagree with your summary.
It's a new behavior for the old renamed module that clearly breaks existing code ported with 2to3.
The 95% of callable usage is to get the estimated download progress (to draw progress bar or time calculations). This requires calculating value of total blocks like:
 total_blocks = math.ceil(float(total_size) / block_size)
when block_size allowed to be 0, it immediately spawns ZeroDivisionErrors. This change broke http://pypi.python.org/pypi/wget
If you make block size in callback parameters varying, the parameter with number of blocks transferred loses any sense.